THE 
EXAMINATION  OF  PROSPECTS 


Published   by  the 

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New  York 

(Successors  to  theBookDepartments  of  tKe 

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Electrical  World  The  Engineering  and  Mining  Journal 

Engineering  Record  American    Mach.ini.st 

Electric  RaiKvay  Journal  Coal  Age 

Metallurgical  and  Chemical  Engineering  Power 

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THE 
EXAMINATION  OF  PROSPECTS 


A  MINING  GEOLOGY 


BY 
C.  GODFREY  CrJJNTHER,  E.  M. 

AUTHOR  OP  "ELECTRO-MAGNETIC  ORE  SEPARATION" 


McGRAW-HILL   BOOK   COMPANY 

239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

1912 


COPYRIGHT,  1912,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY 


PRINTED  AND  ELECTROTYPED 

BY  THE   MAPLE  PRESS 

YORK,    PA. 


PREFACE 

The  purpose  of  this  book  is  to  present  the  practical  side  of 
economic  geology  concisely  and  in  convenient  form;  established 
facts  and  the  applications  of  accepted  views  are  emphasized; 
theoretical  discussions  and  questions  of  genesis  are  avoided. 
Coal,  iron,  and  placer  deposits  are  omitted;  they  are  subjects  of 
specialized  study  that  are  fully  and  concisely  treated  in  other 
works.  The  reader  is  assumed  to  possess  a  knowledge  of  mineral- 
ogy, petrography,  and  of  elementary  geology. 

The  arrangement  adopted  is  based  on  Kemp's  theory  of  mag- 
matic  waters,  Lindgren's  conclusions  on  hydrothermal  and  second- 
ary alterations,  and  on  the  theories  of  secondary  enrichment 
enunciated  by  Emmons  and  Weed. 

No  general  classification  of  ore-deposits  is  attempted,  nor  is 
any  attempt  made  to  fill  by  hypothesis  the  wide  gaps,  nor  to 
explain  the  apparent  contradictions,  that  rank  economic  geology 
among  the  most  inexact  of  sciences.  The  present  knowledge  of 
the  subject  is  too  incomplete  to  warrant  such  broad  generali- 
zations. 

The  present  situation  in  mining  in  the  United  States  may  be 
summed  up  in  the  statement  that  the  demand  for  good  properties 
greatly  exceeds  the  supply.  While  mines  of  the  first  rank  will 
undoubtedly  be  discovered  from  time  to  time,  it  is  probably  true 
that  a  great  proportion  of  deposits  having  outcrops  of  commercial 
grade  or  of  evident  promise  have  been  recognized  and  explored. 
A  review  of  mining  conditions  over  long  periods  shows  that  the 
rich  discoveries  belong  to  pioneer  days,  and  that  as  time  goes  on 
the  more  important  developments  are  the  result  of  lower  working 
costs,  improved  metallurgical  processes,  and  of  an  increasing 
knowledge  of  economic  geology. 

As  engineers  in  search  of  developed  mines  no  longer  expect  to 
find  properties  having  positive  ore  of  greater  net  value  than  the 

251167 


vi  PREFACE 

price  asked,  so  those  in  search  of  prospects  should  not  expect  to 
find  proven  ore-shoots  awaiting  their  recommendation.  There  is 
usually  abundant  local  capital  for  the  preliminary  development 
of  a  patently  good  prospect,  and  most  of  these  are  steadily  worked 
from  the  time  of  their  discovery  until  some  apparently  unfavor- 
able development  shuts  off  the  supply  of  local  capital.  A  great 
majority  of  prospects  have  been  examined  again  and  again,  pre- 
sumably by  men  who  commanded  a  knowledge  of  sampling,  the 
services  of  an  assayer,  and  at  least  an  elementary  knowledge  of 
geology.  In  order  to  pick  a  good  prospect  from  those  rejected 
by  his  predecessors,  therefore,  an  engineer  must  base  his  hope  of 
success  upon  superior  geological  training. 

A  careful  search  has  been  made  of  the  voluminous  bibliogra- 
phy of  economic  geology,  and  the  results  of  this  search  in  great 
part  make  up  the  present  work.  Individual  credit  is  given  in 
foot-notes  wherever  it  may  be  assigned. 

C.  GODFREY  GUNTHER. 

STRATFORD,  CONN.,  June,  1912. 


CONTENTS 

PAGE 

CHAPTER  I.— MIXING  EXAMINATIONS 1 

Formal  examinations — Preliminary  examinations — Examinations 
for  the  rescue  of  badly  expended  capital — The  examination  of  pros- 
pects— The  examination  of  antiguas — Price  and  terms  of  sale — 
The  exploration  of  prospects — Preliminary  search  of  bibliography 
— Apex  and  title — Regularity  of  the  deposit — Condition  of  hang- 
ing wall — Necessity  for  an  accurate  survey — Amount  of  explo- 
ration compared  with  results  attained — Preparing  a  property  for 
examination — A  mine  dressed  for  examination — Past  production 
— Low-grade  ores — Large  versus  small  properties — Sampling — 
The  equipment  for  sampling — Marking  the  samples — Preparation 
to  sample — Placing  the  samples — The  size  of  samples  to  be  taken 
— Salting — The  shipment  of  samples — Resampling — High  assays 
— The  calculation  of  results — Stoping  width — Hand  picking — 
Metallurgical  losses — The  cost  of  mining  and  treatment — The 
estimation  of  ore  reserves. 

CHAPTER  II.— STRUCTURAL  GEOLOGY 26 

Stratification — Cleavage — Fissility — Schistose  structure — Gneissic 
structure — Joints — Sheeting — Folds — The  development  of  faults 
from  folds — Fractures — Flexures — Faults — Exploration  for  the 
continuation  of  a  faulted  ore-body — Post-mineral  fissures — Gauge 
filled  fissures — The  expression  of  faults  in  typography — The  im- 
portance of  aerial  geology — Exploration  and  development. 

CHAPTER  III.— STRUCTURAL  FEATURES  OF  ORE  DEPOSITS 43 

Veins,  lodes  and  ledges — Fissure  veins — Lodes — Ledges — Gash 
veins — Bed  veins — Compound  veins — Contact  veins — '-  Veins  along 
dikes — Lodes  along  sheeted  zones — Stringer  lodes — Fault  lodes — 
The  mineralization  of  joints — Breccia  lodes — Shear  zones — Stock- 
works — Stocks — Pipes  or  chimneys — Branching  veins — Linked 
veins — Conjugate  veins — Overlapping  veins — Systems  of  related 
veins — The  persistence  of  veins  in  depth — The  relation  between 
depth  and  the  number  and  character  of  veins — Mineral  veins  fol- 
low fissures  of  small  displacement — The  influence  of  country  rock 
on  vein  structure — The  distinction  between  intercalated  and 
fissure  veins  in  schistose  rocks — Fissures  and  lodes  formed  subse- 
quent to  the  principal  mineralization. 

vii 


viii  CONTENTS 

PAGE 

CHAPTER  IV. — PRIMARY  ORES  AND  THEIR  DISTRIBUTION 69 

Metallogenetic  epochs  and  provinces — The  distribution  of  ore 
deposits  in  individual  mining  districts — The  association  of  ore 
deposits  with  certain  rocks — The  depth  to  which  primary  ores 
persist — The  criteria  of  primary  ores — The  minerals  of  distinctively 
primary  origin — Distinctively  primary  minerals — Minerals  both 
primary  and  secondary  in  origin — Distinctively  secondary  minerals 
— The  primary  associations  of  metals — The  accessory  minerals 
that  commonly  indicate  a  segregation  of  values — Contact  ores — 
Primary  zinc  ores — The  depth  of  primary  ore  deposition — Deposits 
formed  at  the  surface — Veins  formed  near  the  surface — Veins  of 
deep-seated  origin — Relative  susceptibility  of  hanging  and  foot 
walls  to  mineralization. 

CHAPTER  V. — TYPES  OF  PRIMARY  ORE  DEPOSITS 86 

Magmatic  segregations — Contact  deposits — Pegmatitic  deposits — 
Fahlbands — Regionally  metamorphosed  ore-deposits — Deposits 
due  to  the  filling  of  open  spaces — Replacement  veins — Replace- 
ment deposits — Disseminated  mineralizations — Conglomerate  beds 
— Bedded  ore-deposits — The  Clinton  ore  measures  of  the  United 
States — Red  sandstone  beds  of  the  southwestern  United  States. 

CHAPTER  VI.— PRIMARY  ORE  SHOOTS : 114 

The  factors  that  determine  primary  ore-shoots — The  relative  value 
of  general  and  local  data — Primary  and  secondary  ore-shoots — 
Terms  used  to  describe  the  dimensions  of  ore-shoots — The  shapes  of 
ore-shoots — Lenticular  ore-shoots — The  behavior  of  primary  ore 
shoots  in  depth — The  decrease  in  value  with  depth — Predicting  the 
depth  to  which  ore-shoots  may  continue — The  depths  at  which 
ore-deposits  form — The  structural  features  that  influence  ore 
shoots — Ore-shoots  due  to  available  open  space — Ore-shoots  due  to 
intersections — Ore-shoots  due  to  impounding  of  solutions — Ore 
shoots  at  the  tops  of  anticlines,  or  "saddle  reefs" — The  chemical 
influence  of  wall  rocks  on  ore-shoots — Ore-shoots  in  veins  of  deep- 
seated  origin. 

CHAPTER  VII.— THE  PRIMARY  ALTERATION  OF  WALL  ROCKS.    ...    145 
Metamorphic     process — Dynamo-regional     Metamorphism — Con- 
tact   metamorphism — Hydrothermal    metamorphism — Alteration 
to     greisen     along      tin     veins — Silicifi  cation — Marmorization — 
Dolomitization. 

CHAPTER  VIII. — ALTERATIONS  BY  SURFACE  AGENCIES 153 

The  decomposition  and  weathering  of  rocks — Leaching — Kaolini- 


CONTENTS  ix 

PAGE 

zation — Oxidation — The  migration  and  enrichment  of  metals — 
Upward  migration — The  influence  of  circulation  channels  on  secon- 
dary alterations — The  relation  of  oxidation  and  enrichment  of 
topography  and  water  level — The  irregularity  of  oxidation  and 
enrichment — The  zones  developed  by  surface  agencies — The  depth 
of  vein  leached  to  form  existing  enrichments. 

CHAPTER  IX.— RESIDUAL  ORES  AND  THEIR  DISTRIBUTION.    .     .    .      168 
The  precipitation  of    ores    in  the  zone  of  oxidation — Oxidized 
ores  of  copper — Oxidized  ores  of  lead — Oxidized  ores  of  zinc — 
Residual  shoots  of  gold  ores — The  distribution  of  silver  by  oxi- 
dation— The  distribution  of  manganese  by  oxidation. 

CHAPTER  X. — SECONDARY  ORES  AND  ORE  SHOOTS 173 

The  criteria  of  secondary  ores — Secondary  ore-shoots — The  effect 
of  structural  feature  on  secondary  ore-shoots — The  effect  of  the 
water  level  on  secondary  ore-shoots — The  effect  of  chemically 
active  wall  rocks  on  secondary  ore-shoots — The  effect  of  por- 
osity on  secondary  ore-shoots — The  effect  of  primary  mineralization 
on  secondary  ore-shoots — Ores  containing  both  sulphide  and  oxi- 
dized minerals — The  enrichment  of  copper — Chalcocite  enrichments 
— Disseminated  chalcocite  enrichments — The  enrichment  of  gold 
and  silver  with  copper — The  enrichment  of  silver — The  enrich- 
ment of  gold — The  enrichment  of  lead — The  enrichment  of  zinc 
— The  enrichment  of  lesser  metals — The  migration  of  gangue 
minerals. 

CHAPTER  XI.— OUTCROPS . 196 

The  relation  between  length  of  outcrop  and  persistency  of  vein  in 
depth — The  relation  between  size  of  outcrop  and  width  of  vein  in 
depth — Brecciation  and  post-mineral  fracturing — Meandering  of 
outcrops  on  hillsides — Down-hill  creep — The  topographical  expres- 
sion of  mineralization — Outcrops  of  deposits  formed  at  slight 
depth — Porosity  of  outcrops — Casts  in  resistant  gangue  minerals — 
The  composition  of  outcrops — The  oxides  of  iron  in  gossan — The 
condition  of  outcrops  indicative  of  secondary  enrichments  in  depth 
— Rock  alteration  as  a  guide  to  ore-deposits — The  outcrops  of  kao- 
linized  rocks — The  outcrops  of  contact  deposits — Deposits  of  sur- 
face origin — Microscopical  examination  of  specimens. 


CHAPTER  I 
MINING  EXAMINATIONS 

Mining  examinations  are  of  several  kinds  and  the  scope  of  the 
investigation  depends  in  each  case  upon  the  purpose  for  which 
the  examination  is  made. 

Formal  Examinations. — A  formal  examination  of  a  developed 
mine  is  an  expensive  undertaking;  from  one  to  several  months  are 
allowed  for  the  work  according  to  the  size  of  the  property;  many 
samples  are  taken;  investigation  is  made  of  the  geological  fea- 
tures, including,  perhaps,  a  topographical  and  geological  survey 
of  the  surface;  the  metallurgical  treatment  of  the  ore  is  studied; 
finally,  a  determination  of  costs  is  demanded. 

Formal  examinations  are  made  for  prospective  purchasers  or 
as  a  basis  for  the  consolidation  of  several  properties,  and  occa- 
sionally, on  the  owner's  behalf,  to  verify  the  work  of  resident 
engineers,  or  to  determine  the  readiness  of  a  mine  for  equipment 
and  the  kind  and  capacity  of  equipment  it  shall  receive.  A 
formal  examination  of  a  mine  should  not  be  undertaken  until  a 
preliminary  examination  has  shown  that  it  is  justified. 

It  is  regrettable  that  some  engineers  are  in  the  habit  of  prose- 
cuting long  and  exhaustive  examinations  of  properties  whose 
lack  of  merit  should  be,  and  perhaps  is,  apparent  from  the  start, 
or  of  continuing  their  investigation  long  after  an  unfavorable 
result  is  seen  to  be  inevitable.  This  wasting  of  a  client's  money 
is  a  species  of  trickery  difficult  to  prove,  and  therefore  the  more 
contemptible. 

Preliminary  Examinations. — Preliminary  examinations  are 
precisely  what  the  term  implies,  and  are  undertaken  to  determine 
the  advisability  of  making  a  formal  examination.  There  is 
always  a  reason  why  a  mine  is  offered  for  sale,  which  may  or  may 

1 


OF  PROSP.ECTS 


not  be  kndwn'from  the  start;  frequently  the  chief  object  of  the 
preliminary  examination  is  to  determine  this  reason  for  selling. 

A  preliminary  examination  usually  includes:  the  cutting  of  a 
few  significant  samples,  the  number  depending  upon  the  nature 
and  the  quantity  of  ore  claimed  by  the  owners;  a  geological  recon- 
naissance, usually  involving  a  map;  a  preliminary  study  of  the 
probable  metallurgical  treatment  of  the  ore;  and  a  tentative 
estimate  of  costs.  By  significant  samples  are  meant  samples  of 
the  claimed  ore  reserves,  taken  at  regular  but  longer  intervals 
than  would  be  allowable  in  a  formal  examination,  and  especially 
samples  of  the  lowest  workings,  ends  of  drifts,  and  other  points 
where  a  falling  off  in  value  may  be  suspected.  - 

A  preliminary  examination  should,  of  course,  be  planned  so 
that  the  results  obtained  may  be  supplemented  by,  and  need 
not  be  repeated  in  the  formal  examination  that  may  follow. 
Data  that  is  sufficiently  reliable  for  use  in  a  formal  report  is 
necessary  for  a  reliable  preliminary  report;  the  work  should  not 
be  slighted  because  it  is  to  be  checked  if  it  yields  a  satisfactory 
result. 

Examinations  for  the  Rescue  of  Badly  Expended  Capital. — It 
frequently  happens  that  an  engineer  is  called  in  only  after  funds 
are  exhausted,  ore  reserves  depleted,  metallurgical  processes 
failures,  and  ruin  appears  inevitable.  This  is  a  situation  that 
requires  special  aptitude;  valuable  clients  are  at  stake  as  well  as 
the  recovery  of  a  part  of  the  ill-advised  outlay;  furthermore, 
what  under  the  circumstances  may  be  considered  a  flattering- 
salvage  is  almost  certain  to  appear  small  to  the  owners  after  the 
glowing  promises  of  the  mistaken  promoters. 

In  examinations  of  this  kind  the  work  should  be  concentrated 
upon  the  most  favorable  showings,  a  careful  review  of  the  metal- 
lurgical processes  should  be  made,  and,  in  fact,  every  loophole 
that  promises  salvage  should  be  investigated.  Where  large 
bodies  of  low  grade,  unpayable  ore  have  already  been  developed, 
hand  picking  should  be  tried  in  the  hope  of  securing  a  com- 
mercial product. 

The  failure  having  perhaps  been  due  to  haphazard  exploration, 


MINING  EXAMINATIONS  3 

the  geological  relations  of  the  ore  should  be  carefully  deter- 
mined, and  all  stopes  should  be  surveyed,  as  it  is  probable  that 
the  best  ore  was  mined  and  that  nothing  really  good  was  left. 
This  data,  together  with  assay  plans  of  the  existing  ore  and  all 
geological  features,  should  be  entered  upon  a  large-scale  map, 
upon  which  relations  will  become  clear  that  otherwise  would  not 
suggest  themselves.  A  little  intelligent  exploration  based  upon 
such  a  map  will  in  many  instances  yield  important  results. 

The  Examination  of  Prospects. — The  examination  of  a  prospect 
is  a  very  different  undertaking  from  the  examination  of  a  mine; 
prospec'ts  are  not  expected  to  show  ore  reserves  as  a  basis  for 
purchase,  and  in  the  last  analysis  the  recommendation  of  a 
prospect  rests  on  an  opinion  rather  than  demonstrable  facts. 

The  examination  of  a  prospect  requires  that  all  significant 
samples  shall  be  taken  and  a  thorough  geological  investigation  be 
made,  which  need  not,  however,  be  put  in  formal  shape  unless  it 
yields  a  favorable  result.  The  question  to  be  answered  in 
examining  a  prospect  is:  "  What  chance  has  it  to  make  a  mine?" 
that  a  prospect  be  acquired  under  option  as  is  justifiable  before 

The  same  hesitation  should  not  be  felt  in  recommending 
advising  the  purchase  of  a  developed  mine.  It  should  be 
remembered  that  the  majority  of  prospects  have  been  examined 
many  times,  and  that  no  brilliant  showing  of  payable  ore  will  be 
encountered;  any  fairly  consistent  showing  of  geological  promise 
is  worth  considering,  and  merits  the  expenditure  of  a  few  hun- 
dreds or  a  few  thousands  of  dollars  in  preliminary  work,  which, 
if  it  yields  a  favorable  result,  may  be  followed  by  more  serious 
development. 

Mining  booms,  even  if  founded  on  mistaken  estimates,  fre- 
quently lead  to  the  discovery  of  valuable  properties;  this  is  due 
to  the  fact  that  the  excitement  of  a  boom  leads  to  much  shallow 
prospecting,  which  often  exposes  conditions  not  evident  from  an 
inspection  of  the  immediate  surface.  Test_pits  and  trenches 
may  be  considered  as  halfway  between  examination  work  and 
exploration,  and  while  inexpensive,  they  not  infrequently  yield 
important  results. 


4  EXAMINATION  OF  PROSPECTS 

The  Examination  of  Antiguas. — The  presence  of  ancient  work- 
ings should  not  be  assumed  to  indicate  a  valuable  property. 
Those  that  have  had  much  to  do  with  antiguas  in  Mexico  have 
respect  for  the  ability  of  the  pioneers  to  follow,  to  extract,  and  to 
treat  ores.  These  properties  were  worked  with  slave  labor;  a 
certain  proportion  of  the  force  being  detailed  to  grow  food  for 
the  miners,  the  labor  cost  of  mining  was  practically  nil;  very  low 
grade  material,  therefore,  could  be  mined,  as  almost  any  recovery 
from  the  ores  was  profit.  Similar  conditions  probably  obtained 
wherever  mines  were- operated  by  the  Spanish,  or  by  the  ancients. 

No  antigua  should  be  expected  to  show  high-grade  ore;  the 
Mexicans,  for  example,  are  expert  miners,  and  will  be  found  to 
have  removed  all  pillars  or  other  payable  ore  that  was  left  by  the 
pioneers  in  any  property  now  sufficiently  open  for  examination. 

The  first  step  in  the  examination  of  an  antigua  is  to  make  a 
survey  and  to  map  all  the  workings,  placing  on  the  map  all  the 
geological  data  obtainable.  The  pioneers  did  not  understand 
faults,  and  never  drove  exploratory  cross-cuts;  not  infrequently 
a  detailed  geological  study  of  an  antigua  results  in  finding  im- 
portant ore-bodies. 

By  means  of  samples,  the  grade  of  ore  left  in  the  mine  by  the 
pioneers  can  be  ascertained,  and  this  will  often  determine 
whether  or  not  the  property  may  be  expected  to  yield  a  profit 
with  the  application  of  modern  metallurgical  processes;  all  ore 
above  this  minimum  grade  may  be  assumed  to  have  been 
extracted. 

The  pioneer  miners  could  not  follow  the  ore  very  far  below 
water  level,  and  any  antigua,  the  ore  in  which  is  primary,  offers 
an  attractive  opportunity;  in  most  cases,  however,  the  ores  mined 
were  secondary,  and  probably  do  not  continue  far  below  the  old 
workings  before  changing  to  low-grade  primary  material. 

In  the  examination  of  antiguas  the  mode  of  bringing  the  ore  to 
the  surface  should  be  carefully  considered.  The  pioneers  raised 
it  on  men's  backs,  and  their  workings — following  the  ore — are 
tortuous.  It  sometimes  proves  cheaper,  where  the  tonnage  is 
small,  to  pursue  this  method;  the  cost  of  a  straight  shaft  and 


MINING  EXAMINATIONS  5 

hoisting  equipment  must  be  apportioned  against  each  ton  of  ore, 
and  modern  equipment  may  prove  in  the  end  more  expensive 
than  the  ancient  method. 

If  the  pay  shoots  in  an  antigua  are  short  and  the  stopes 
narrow,  the  chfrces  are  greatly  against  a  profitable  outcome  for 
further  exploration. 

Price  and  Terms  of  Sale. — The  final  object  of  a  mining  exam- 
ination being  a  profit  for  the  client  for  whom  the  work  is  done, 
the  price  and  terms  asked  are  considerations  second  to  none;  in 
the  United  States  they  are  too  often  but  slightly  considered,  and 
sales  of  undeveloped  properties  are  made  at  prices  that  largely 
discount  even  a  decidedly  favorable  outcome  for  the  proposed 
development. 

A  great  majority  of  mining  examinations  lead  to  unfavorable 
reports.  This  is  owing  to  the  fact  that  the  demand  for  good 
properties  greatly  exceeds  the  supply,  and  to  the  great  difference 
between  the  points  of  view  of  the  owners  of  mining  properties 
and  the  examining  engineers.  The  owners  generally  suffer  from 
extreme  optimism,  and  many  engineers  from  excessive  profes- 
sional timidity,  and  neither  is  willing  to  meet  the  other  halfway. 
Owners  of  prospects  are  usually  brought  to  their  senses  after 
repeated  unfavorable  examinations,  but  many  engineers  never 
make  a  favorable  report  because  of  the  risk  of  personal  reputation. 
Mining  is  essentially  a  risky  business,  and  he  who  declines  to 
accept  some  risk  will  not  make  money  for  his  clients. 

No  engineer  should  expect  to  find  a  mine  having  ore  of  a 
greater  net  value  than  the  purchase  price  asked,  unless  the  mine 
is  admittedly  bottomed,  and  has  no  possibilities  beyond  the  ore 
already  developed;  this  is  a  rare  case,  and  most  developed  mines 
in  addition  to  their  ore  reserves  hold  a  certain  promise  for  further 
tonnages  of  payable  ore;  each  case  must  be  considered  on  its  own 
merits,  and  how  much  to  allow  for  the  value  of  prospective  ore 
will  differ  with  every  engineer. 

Mr.  J.  H.  Curie1  gives  as  his  rule  that  66  per  cent,  of  the  pur- 
chase price  should  be  represented  in  net  value  of  ore  reserves, 
Gold  Mines  of  the  World,"  p.  49. 


6  EXAMINATION  OF  PROSPECTS 

with  the  lower  levels  still  looking  well.  On  the  basis  of  66 
per  cent,  of  the  purchase  price  to  be  represented  in  ore  reserves, 
most  sales  are  made  at  too  high  a  figure,  which  is  borne  out 
by  the  fact  that  most  mining  ventures  are  not  profitable,  the 
balance  being  established  by  the  occasional  large  success  that 
repays  its  capital  many  times  over.  These  considerations  apply 
chiefly  to  the  sales  of  properties  in  an  advanced  stage  of  develop- 
ment and  not  to  prospects,  with  which  this  work  chiefly  treats. 

The  Exploration  of  Prospects. — In  the  exploration  of  prospects 
it  should  be  expected  that  many  ventures  will  prove  failures,  as 
the  exceptional  success  will  more  than  repay  a  number  of  unsuc- 
cessful ventures.  The  exploration  of  prospects  should  be 
undertaken  only  when  the  operators  have  abundant  capital,  and 
the  pertinacity  to  acquire  and  explore  several  properties  suc- 
cessively. In  such  a  campaign  it  is  evident  that  the  costs  of 
each  exploration  should  be  kept  as  Tow  as  possible,  and  that 
exploration  should  cease  as  soon  as  the  chances  of  success  are 
reduced  below  the  apparent  chances  when  the  property  was 
acquired  for  exploration;  persistency  beyond  a  certain  point  is 
a  failing. 

In  order  to  keep  the  costs  low,  cash  payments  should  not  be 
made  on  prospects,  nor  should  expensive  machinery  be  installed, 
as  such  expenditures  in  an  enterprise  of  this  sort  must  be  charged 
against  the  footage  of  work  done.  It  is  almost  always  advisable 
at  the  start  to  follow  the  ore,  and  with  as  cheap  temporary 
equipment  as  will  answer  the  immediate  purpose. 

While  it  is  true  that  the  speculative  chance  held  by  a  good 
looking  prospect  has  a  certain  cash  value,  only  in  rare  cases 
should  a  cash  payment  be  made;  a  monthly  rental,  or  a  "salary" 
to  the  owner,  during  the  period  of  exploration,  frequently  offers 
a  convenient  compromise. 

Preliminary  Search  of  Bibliography. — If  an  examination  is  to 
be  made  in  an  established  district,  a  search  of  the  bibliography 
of  the  district  is  an  important  preliminary  to  the  examination. 
Written  statements  are  more  likely  to  be  accurate  and  conserva- 
tive than  spoken  statements,  and  important  data  are  frequently 


MINING  EXAMINATIONS  7 

obtainable.  If  the  library  is  well  indexed,  a  search  of  this  kind 
is  not  a  long  undertaking. 

Apex  and  Title. — The  questions  of  extralateral  rights  and  title 
are  of  prime  importance  in  the  United  States.  A  preliminary 
investigation  by  the  examining  engineer  usually  consists  in 
questioning  residents.  In  the  event  that  a  sale  is  to  be  made, 
these  points  should  be  investigated  by  a  lawyer.  The  slightest 
chance  of  legal  entanglement  is  a  sufficient  basis  upon  which  to 
drop  a  negotiation,  for  if  a  mine  proves  successful,  the  number 
and  backing  of  the  contestants  will  be  .multiplied. 

Regularity  of  the  Deposit. — The  regularity  of  a  deposit  together 
with  its  size  determines  the  cost  of  development.  Of  two  depos- 
its that  contain  a  like  quantity  of  payable  ore,  the  regular  deposit 
is  by  far  the  more  valuable. 

Unless  the  development  of  a  mine  is  complete,  which  is  rarely 
the  case,  the  amount  of  ore  it  contains  is  not  known  until  after 
it  is  worked  out.  A  regular  deposit  is  cheaply  and  rapidly 
developed  and  its  value  may  be  clearly  indicated  and  realized 
through  sale;  a  majority  of  irregular  deposits,  however,  probably 
do  not  repay  the  cost  of  the  development  necessary  to  indicate 
their  value,  and  must  be  worked  by  following  and  extracting 
the  ore  as  it  is  found.  An  example  of  this  type  of  property  is 
the  Butte  Mine  at  Randsburg,  California,1  which  has  produced 
$525,000,  although  it  is  said  not  at  any  time  to  have  had  more 
than  $5000  worth  of  ore  exposed;  many  lead  deposits  in  lime- 
stone are  of  this  type. 

Examples  are  numerous  of  deposits  whose  regularity  permits 
the  working  of  very  low-grade  material :  the  so-called  "  porphyry- 
copper"  deposits  are  of  this  class,  as  are  also  deposits  of  the  type 
of  the  Alaska  Treadwell;  the  regularity  of  these  deposits  con- 
sidered as  a .  whole  (they  are  locally  quite  irregular)  permits 
cheap  development,  which  in  turn  permits  the  construction  of 
the  large  plants  necessary  to  treat  such  low  grades  of  ore. 

Certain  mines  in  the  Catalina  Mountains,  Arizona,  are  good 
examples  of  large  but  irregular  deposits;  these  properties  have 

lBull.  430,  U.  S.  G.  S.,  p.  40. 


8  EXAMINATION  OF  PROSPECTS 

produced  large  quantities  of  payable  ore,  but  at  no  time  was 
sufficient  ore  exposed  to  warrant  the  construction  of  a  16-mile 
railroad  connection,  or  a  smelting  plant:  as  a  result,  the  ore 
produced  must  be  hauled  to  the  railroad  in  wagons  and  shipped 
to  a  custom  smelter  at  heavy  cost,  and  the  mines  are  intermit- 
tently operated. 

Condition  of  Hanging-Wall. — A  strong,  firm  hanging-wall  is 
not  infrequently  a  controlling  factor  in  the  operation  of  large, 
low-grade  deposits,  where  the  cost  of  timbering  cannot  be  borne 
by  the  ore.  The  Granby,  and  other,  mines  in  the  Boundary 
District  of  British  Columbia,  probably  could  not  operate  if  it 
'were  not  for  the  fact  that  their  large  stopes  remain  open  without 
artificial  support:  the  mines  of  Douglas  Island,  Alaska,  are 
other  examples,  and  the  firm  hanging-wall  is  a  factor  of  the 
greatest  importance  in  the  success  of  the  Pilares  Mine,  at  Nacozari, 
Sonora,  and  of  many  other  properties  that  contain  large  deposits 
of  low-grade  ore.  Most  of  the  so-called  "porphyry-copper" 
deposits  have  to  contend  with  heavy  hanging-walls  of  'soft, 
altered  porphyry,  and  this  disadvantage,  it  is  thought,  will 
become  increasingly  apparent  as  underground  mining  is  con- 
ducted on  a  large  scale. 

Necessity  for  an  Accurate  Survey. — A  survey  and  map  are 
imperative  to  a  proper  understanding  of  any  but  the  simplest 
occurrence  of  ore.  All  the  geological  data  should  be  placed  on 
the  map,  without  which  they  are  likely  to  be  unintelligible.  For 
a  preliminary  survey,  work  done  with  a  Brunton,  or  other  pocket 
transit,  is  of  sufficient  accuracy.  The  writer  has  made  satis- 
factory maps  without  an  instrument,  using  a  straightedge  on  a 
large  sheet  of  paper  fastened  to  a  smooth  surface,  afterward 
taking  off  the  angles  with  a  straightedge  and  triangle. 

Refractory  Ores. — That  an  ore  is  refractory  is  often  apparent 
on  simple  inspection,  as,  for  instance,  a  lead-silver  ore  that 
carries  much  zinc,  or  copper  in  an  ore  for  which  cyanidation  would 
be  natural  treatment.  A  general  knowledge  of  metallurgy  is 
necessary  to  the  examining  engineer,  and  while  a  discussion  of  the 
details  and  costs  of  treatment  are  out  of  place  in  a  report  on  a 


MINING  EXAMINATIONS  9 

prospect,  the  amenability  of  the-  ore  to  metallurgical  treatment 
should  be  borne  in  mind. 

In  the  examination  of  a  property  the  ore  from  which  is  to  be 
concentrated,  the  probable  ratio  of  concentration  should  be  con- 
sidered: if  the  valuable  constituent  occurs  without  accessory 
heavy  metals,  the  ratio  will,  of  course,  be  high;  if  the  ore  carries 
a  large  quantity  of,  for  example,  barren  pyrite  in  addition  to  the 
valuable  constituent,  the  ratio  of  concentration  will  be  low,  as 
will  be  also  the  grade  of  the  concentrates.  This  condit'on  is 
said  to  be  a  source  of  disappointment  in  certain  large  copper 
mines  in  Nevada. 

Furthermore,  ores  whose  valuable  metals  are  present  in  both 
oxidized  and  sulphide  form  are  difficult  to  concentrate,  because 
of  the  widely  differing  specific  gravities  of  their  valuable 
minerals. 

In  an  ore  in  which  a  part  of  the  valuable  metal  is  contained  in 
soluble  form,  the  amount  so  present  must  be  considered  a  net 
loss,  and  the  assays  must  be  correspondingly  diminished:  an 
example  of  this  condition  that  is  often  met  in  arid  regions  is  a 
copper  ore  of  concentrating  grade  that  contains  a  part  of  its  cop- 
per as  chalcanthite;  where  such  an  ore  exists  in  large  deposits, 
leaching  may  offer  a  profitable  method  of  treatment. 

Amount  of  Exploration  Compared  with  Results  Attained. — The 
relation  between  the  amount  of  work  done  and  the  quantity  of 
ore  exposed  is  an  important  one;  a  given  quantity  of  ore  being 
exposed,  the  result  may  be  considered  satisfactory  or  unsatis- 
factory according  to  whether  little  or  much  exploration  was 
necessary  to  develop  it. 

In  levels  from  a  timbered  shaft  it  is  important  to  note  whether 
they  were  driven  at  regular  or  irregular  intervals;  if  the  latter, 
it  is  more  than  likely  that  the  widest  and  best  parts  of  the  deposit 
were  selected  in  which  to  drive  the  levels,  and  that  they  do  not, 
therefore,  indicate  the  average  character  of  the  deposit. 

Preparing  a  Property  for  Examination. — This  is  a  subject  that 
the  owners  of  mining  properties  would  do  well  to  study.  The 
fact  that  the  surface  offers  a  complete  section  of  the  ore-deposits 


10  EXAMINATION  OF  PROSPECTS 

and  of  their  surroundings  is  often  lost  sight  of;  before  attempting 
deeper  work  it  is  well  to  explore  the  surface  thoroughly  with 
trenches  and  test  pits.  If  no  ore  is  found  by  this  work,  and  no 
indication  found  of  residual  conditions  suggesting  that  ore 
once  existed  at  this  level,  then  deeper  exploration  is  not 
warranted. 

Where  the  ore  or  gangue  mineral  is  harder  and  more  resistant 
to  weathering  than  the  enclosing  rock,  the  outcrops  are  usually 
bold  and  but  little  surface  work  is  necessary,  but  where  the  ore 
or  gangue  is  softer  than  the  enclosing  rocks,  the  veins  or  ore-bodies 
do  not  outcrop,  and  surface  workings  are  necessary  to  expose  the 
deposits  for  examination. 

The  surface  is  always  a  fair  criterion  of  the  conditions  in  depth, 
if  interpreted  in  the  light  of  the  present  knowledge  of  impover- 
ishment and  enrichment  due  to  surface  agencies. 

Development  should  follow  the  ore  or  vein;  there  is  nothing 
more  provoking  than  to  be  asked  to  examine  a  good  surface 
showing  somewhere  beneath  which  a  "cross-cut"  tunnel  has 
been  driven  that  throws  no  light  on  the  conditions  at  that  depth, 
but  inferentially  discourages  further  work. 

In  preparing  a  mine  for  examination,  the  ore  should  be  cross- 
cut to  its  full  width;  no  engineer  will  allow  for  ore  that  remains 
"in  the  wall." 

A  Mine  Dressed  for  Examination. — A  mine  that  has  been 
dressed  for  examination  may  be  described  as  a  trap  for  the  ex- 
amining engineer.  The  favorable  features  are  accentuated,  and 
the  unfavorable  developments  are  concealed, — drifts  are  walled 
up,  or  are  allowed,  or  encouraged,  to  cave,  winzes  are  allowed  to  fill 
with  water,  and  workings  through  poor  stretches  of  the  deposit 
are  tightly  timbered  to  hinder  examination.  In  stoping  opera- 
tions barren  material  is  removed  up  to  the  best  showings,  which 
are  left  undisturbed,  on  the  supposition  that  the  engineer  will 
assume  that  the  material  stoped  was  payable  ore.  Exploratory 
drifts  are  frequently  stopped  in  good  ore;  it  is  astonishing  in  how 
many  cases  a  man  who  is  familiar  with  the  local  ore-shoots  can 
stop  his  drift  just  short  of  running  into  barren  ground. 


MINING  EXAMINATIONS  11 

Vigilance  and  a  suspicious  attitude  are  the  engineer's  safe- 
guards against  this  kind  of  fraud.  A  method  followed  by  the 
writer  is  almost  sure  to  result  in  detection  if  trickery  has  been 
attempted.  On  going  through  the  mine  many  questions  are 
asked  in  regard  to  every  point  that  suggests  itself,  and  the 
answers  are  set  down.  The  questions  are  repeated  and  answers 
noted  from  as  many  of  the  chief  men  on  the  ground  as  may  be 
induced  to  answer  them.  After  an  interval  the  questions  are 
repeated,  but  not  in  the  same  order,  and  the  answers  are  again 
noted.  Before  leaving  the  property  a  third  inquisition  will 
yield  the  desired  result.  If  the  local  representatives  have  been 
telling  the  truth,  their  answers  will  check  up,  but  it  is  a  very 
exceptional  liar  that  can  stick  to  a  fabric  of  falsehoods  three 
separate  times  with  long  intervals  between. 

Past  Production. — A  study  of  smelter  or  mill  returns  from  past 
ore  shipments  in  connection  with  the  amount  of  exploration  done 
is  often  instructive  in  the  examination  of  irregular  deposits. 
The  average  value  of  past  ore  shipments,  of  course,  is  no  criterion 
of  the  grade  of  ore  left  in  the  mine,  as  the  best  ore  is  invariably 
extracted  first. 

Low -Grade  Ores. — A  slightly  explored  property  carrying  low- 
grade  ore  that  may  be  expected  just  to  pay  its  way  should  be 
explored,  as  higher-grade  ore  may  be  found.  Any  large  deposit 
of  even  very  low-grade  material  should  be  given  at  least  a  pre- 
liminary examination,  as  the  constant  improvement  in  metal- 
lurgical processes  is  steadily  rendering  payable  ores  of  lower 
and  lower  grade.  The  waste  of  15  years  ago  is  the  ore  of  to- 
day, and  the  same  advance,  may  be  reasonably  expected  in  the 
future. 

Large  Versus  Small  Properties. — In  general,  a  large  body  of 
low-grade  ore  is  more  likely  to  persist  than  a  smaller  body  of 
high-grade  ore.  Small  mines,  unless  containing  shipping  ore 
and  requiring  little  equipment  (and  these  are  rarely  purchasable 
•at  a  reasonable  price),  are  usually  not  profitable.  The  cost  of 
all  equipment  must  in  the  final  analysis  be  charged  against  the 
tonnage  of  ore  extracted,  and  one  mine  may  yield  a  handsome 


12  EXAMINATION  OF  PROSPECTS 

profit  where  two  mines  each  containing  one-half  the  tonnage  of 
the  same  grade  may  both  net  losses.  Furthermore,  a  small  prop- 
erty is  in  a  poor  strategic  position  to  secure  good  smelting  or 
freight  rates. 

Sampling. — A  discussion  of  sampling,  although  fully  treated 
elsewhere/  cannot  logically  be  omitted;  inasmuch  as  proper 
sampling  is  often  neglected,  the  subject  will  bear  repetition.2 
Sampling  is  expensive  work  if  properly  carried  out,  and  no  other 
kind  of  sampling  is  of  any  value. 

The  ideal  sample  is  a  uniform  groove,  or  channel,  across  the 
full  width  of  the  ore,  and  no  more;  how  closely  this  may  be 
approached  in  practice  will  depend  upon  the  material  sampled 
and  upon  the  time  and  care  given  to  the  work. 

The  Equipment  for  Sampling. — A  hammer  and  moil  are  prefer- 
able to  any  other  tools  in  cutting  samples;  a  prospector's  pick 
will  do  good  work  in  soft,  uniform  ground,  but  in  harder  material, 
even  if  the  point  and  hammer-end  are  alternately  used,  is  likely 
to  have  a  selective  effect,  and  samples  taken  with  a  pick  are  not 
above  suspicion. 

To  catch  the  sample  a  cloth  is  best,  spread  out  so  as  to  catch 
all  chips.  If  the  ground  is  loose  and  masses  are  likely  to  fall, 
the  sample  is  best  caught  in  a  box,  which  is  also  used  where  fine, 
rich  material  is  likely  to  sift  out  of  cracks  and  vugs  and  so  find 
its  way  into  the  sample. 

To  break  down  samples  a  crusher  is  convenient,  but  two  large, 
tough  stones  of  barren  rock  such  as  may  be  found  in  any  creek 
bed,  one  to  lay  the  ore  upon  and  the  other  to  pound  with,  yield 
the  maximum  result  for  coarse  breaking;  unless  the  ore  is  very 
hard,  the  abrasion  of  the  stones  is  negligible. 

To  cut  down  samples,  rolling  and  quartering  on  flexible  oil- 
cloth is  a  good  method,  but  where  many  samples  are  to  be  taken, 

l"  Sampling  and  Estimation  of  Ore  in  a  Mine,"  T.  A.  Rickard  and 
others. 

2  Credit  cannot  be  given  to  all  those  that  have  written  on  this  subject 
for  the  many  practical  points  that  come  up  as  obvious  solutions  of  the 
minor  problems  of  mining  examinations. 


MINING  EXAMINATIONS 


13 


a  Jones  sampler  with  four  pans,  or  a  riffle,  is  quicker  and  more 
certain  to  do  accurate  work. 

A  simple  apparatus  to  permit  the  cutting  of  samples  in  an 
untimbered  shaft  has  been  used  by  the  writer  with  great  saving 
of  time  and  expense  over  erecting  platforms.  This  consists  of  a 
short  seat,  about  14  in.  long  by  6  in.  wide,  to  which  is  rigidly 
fastened  a  pole  in  length  about  one  and  one-half  times  the  width 
of  the  shaft  to  be  sampled.  This  seat  is  fastened  to  the  end  of 
the  windlass  rope,  and,  straddling  the  rope,  the  sampler  is  lowered 
to  the  point  where  a  sample  is  to  be  taken,  meanwhile  holding 
the  pole  parallel  to  the  rope  so  as  to  permit  the  descent.  Arriving 
at  the  point  where  the  sample  is  to  be  taken,  the  pole  is  allowed 
to  fall  against  the  opposite  I — • 

wall,  the  end  of  the  pole  catch      Notch  A 
ing   in    an  inequality  of  the 
rock,  the  seat  is  hoisted  a  few 
inches,  and  the  sampler,  with 
feet   braced  against  the  face 

sampled,  is  held  firmly  and  ^=^^        Side  View 

safely  in  position,  with  both        FIG.   1.— Sketch  showing  sampling 

handsfreeto  work.     The  sam-   seat  f°r  ^VV^^f  f^  ^ 
,  ,    .  pole  should  be  bolted  firmly  to  the  seat, 

pie  may  be  caught  in  a  bag   J 

held  between  the  feet,  or  a  canvas  receptacle  may  be  rigged  in 
front  of  the  sampler,  or  the  sample  may  be  allowed  to  fall  on  a 
canvas  spread  over  the  bottom  of  the  shaft,  which  should  be 
protected  from  the  impact  of  the  falling  ore  by  a  few  boards; 
the  latter  is  usually  not  a  safe  method  unless  the  men  at  the  wand- 
lass  are  closely  watched,  as  salting  by  them  would  be  an  easy 
matter. 

Marking  the  Samples. — Durable  tags  carrying  the  number  of 
the  sample  should  be  inserted  in  each  sack;  some  engineers  use 
metal  tags,  some  wood,  but  the  usual  practice  is  to  use  tough 
paper  rolled  up  tightly  to  prevent  abrasion.  The  following 
sample  tag  is  excellent,  and  should  be  made  up  in  books  contain- 
ing 50  sheets  and  numbered  before  going  into  the  mine.1  The 

1  R.  C.  Gemmel,  "Sampling  and  Estimation  of  Ore  in  a  Mine,"  p.  185. 


14 


EXAMINATION  OF  PROSPECTS 


lower  part  of  the  tag  is  torn  off,  and  after  being  rolled  up,  is 
inserted  in  the  sack  'with  the  sample.  Tearing  the  detached  slip 
in  half  gives  two  numbers  for  the  duplicates  when  the  sample  is 


Date. 

Sample  taken 

At  point 

From 

Across 

For Ft In 

Measurement:  At  right  angles  to  dip,  vertical, 
horizontal. 

To 

Dip Strike 

No 

(perforation  here) 

No 

No..  , 


cut  down.  On  the  backs  of  these  slips  the  writer  is  accustomed 
to  make  a  sketch  in  section  of  the  place  sampled,  showing  the 
shape  of  the  drift  and  the  sample  cut  by  a  dotted  line;  the 
geological  features  may  also  be  indicated  with  colored  pencils;  and 
any  remarks  noted.  The  data  is  thus  kept  in  convenient  form 
for  future  reference  and  is  invaluable  in  case  of  dispute;  it  fre- 
quently happens  that  an  unfavorable  report  is  questioned  or 
disputed,  and  in  any  controversy  regarding  samples  these  books 
so  kept  will  put  the  adversary  to  rout  on  sight. 

The  writer  places  th$  sample  number  on  the  outside  of  the 
sacks  to  permit  ready  identification  without  having  to  pour  out 
the  sample  in  search  of  the  tag.  This  practice  is  objected  to  by 
some  engineers  on  the  ground  that  it  permits  an  outsider  to  locate 
the  samples  with  equal  ease.  If  an  outsider  gets  close  enough  to 
the  samples,  and  the  leisure  in  which  to  inspect  the  numbers,  he 
is  close  enough  to  tamper  with  them,  and  this  objection  is  not, 
therefore,  considered  as  having  weight. 

Preparation  to  Sample. — Samples  should  be  accurately  referred 
to  some  permanent  object,  such  as  a  cross-cut,  winze,  or  survey 


MINING  EXAMINATIONS  15 

station.  The  intervals  between  samples  should  be  measured 
along  the  center  of  the  drift,  as  they  otherwise  will  differ  widely 
according  as  they  are  measured  on  one  side  of  the  drift  or  the 
other,  and  will  therefore  fail  to  plot  correctly  on  the  map. 

It  is  poor  judgment  to  mark  the  points  at  which  the  samples 
are  to  be  taken  in  advance  of  the  actual  sampling;  this  amounts 
to  an  advertisement  that  a  sample  is  to  be  taken  along  a  certain 
line,  and  permits  the  evilly  disposed  to  assist  nature  in  the  dis- 
tribution of  values. 

A  face  that  is  to  be  sampled  should  be  thoroughly  cleaned.  If 
the  ground  is  soft,  a  strip  a  few  inches  wider  than  the  sample  cut 
should  be  cleaned  off  with  a  pick;  if  the  ore  is  hard,  a  brush  or 
broom  should  be  used  either  dry  or  with  water.  In  driving  any 
working,  the  fine  material,  often  the  richest,  is  powdered  and 
thrown  against  the  roof  and  walls,  where  a  portion  of  it  adheres; 
it  is,  therefore,  of  the  greatest  importance  that  the  face  to  be 
sampled  should  be  thoroughly  cleaned.  Irregular  projections 
and  loose  pieces  should  be  knocked  off,  to  give,  in  so  far  as 
possible,  a  flat  surface  from  which  to  cut  the  sample. 

The  face  to  be  sampled  should  be  examined  carefully  for  soluble 
salts.  In  copper  mines  in  dry  climates  an  efflorescence  of  chal- 
canthite  and  other  salts  is  usual,  and  the  sampling  of  old  workings 
is  attended  with  considerable  risk  of  salting  from  this'  cause. 
Samples  containing  these  efflorescences,  even  after  boiling  in 
water,  show  blue  crystals  under  the  microscope.  These  efflores- 
cences are  due  to  the  evaporation  of  migrating  solutions  on  the 
walls  of  the  workings,  and  represent  an  enrichment  not  found 
throughout  the  mass  of  rock.  The  writer  had  occasion  to  re- 
sample  a  mine  where  there  was  much  efflorescence.  His  samples 
were  boiled  in  water  with  a  little  caustic  soda  and  averaged  about 
3/10  per  cent,  copper;  the  sampling  thus  discredited  averaged  in 
excess  of  2  per  cent. 

Where  it  is  necessary  to  take  samples  from  the  floors  of  drifts 
it  is  best  to  cut  large  samples  and  to  w^ash  from  them  and  dis- 
regard all  fine  material;  fine  particles  of  heavy  minerals  work  into 
cracks  'in  the  floor  and  give  deceptively  high  results.  This 


16 


EXAMINATION  OF  PROSPECTS 


method  may  give  results  somewhat  too  low,  but  is  not  so  liable 

to  serious  error  as  would  result  from  the  inclusion  of  the  fine 

material. 

Samples  shoulji  always  be  taken  as  nearly  as  possible  at  right 

angles  to  the  lines  of  distribution  of  the  minerals  through  the  ore. 

Placing  the  Samples. — The 
interval  between  samples  de- 
pends upon  the  regularity  with 
which  the  valuable  minerals 
are  distributed.  One  extreme 
might  be  considered  an  abso- 
lutely uniform  mass,  of  which 
one  sample  would  suffice,  and 
the  other  extreme,  a  segrega- 
tion of  all  the  valuable  mineral 
into  a  single  mass;  it  is  there- 
fore apparent  that  the  proper 
interval  between  samples  will 
differ  with  each  exposure 
sampled. 

In  general,  a  20-ft.  interval 
will  suffice  for  a  large  ore-shoot 
of  uniform  grade,  a  10-ft.  in- 
terval in  average  cases  where 
many  samples  are  to  be  taken 
from  the  same  ore-shoot,  and 
5-ft.  or  lesser  intervals  where 


FIG  2. — Cross  section  showing  ir- 
regular exposure  of  ore  in  the  roof  of  a 
drift;  the  quantity  of  sample  taken  per 
foot  should  be  less  along  A-B  and  C-D 
than  along  B-C,  where  the  roof  is  ap- 
proximately at  right  angles  to  the  vein. 


the  ore  is  spotty.  It  is  usually 
advisable  to  start  with  20-ft.  intervals  in  the  examination  of 
a  property  where  no  data  are  available,  and  resample  at  10-ft. 
intervals,  and  perhaps  again  at  5-ft.  intervals,  where  the  results 
from* the  first  series  indicate  that  such  a  course  is  advisable. 

In  sampling  a  wide  vein  or  deposit  it  is  best  to  divide  the 
width  into  sections  and  to  sample  them  separately,  in  order  to 
determine  the  distribution  of  the  values.  These  widths  may  be 
taken  over  even  multiples  of  the  total  width  if  the  deposit  pre- 


MINING  EXAMINATIONS  17 

sents  a  uniform  appearance.  If  the  vein  or  deposit  presents  a 
variegated  or  banded  appearance,  however,  the  several  bands  or 
zones  should  be  sampled  separately. 

Where  a  section  of  ore  is  irregularly  exposed,  as  is  commonly 
the  case  with  a  vein  in  the  roof  of  a  drift,  the  sample  must  be  cut 
deeper  over  the  part  that  is  at  right  angles  to  the  vein  than  where 
the  face  is  slanting,  in  order  not  to  get  an  undue  proportion  from 
the  slanting  exposure. . 

The  Size  of  Samples  to  be  Taken. — The  size  of  a  sample  should 
be  limited  to  the  least  amount  that  will  yield  a  true  average  of 
the  exposure  sampled.  A  few  large  samples  are  of  little  value 
as  compared  with  many  smaller  samples,  if  the  latter  be  well 
taken.  Car-load  shipments,  ton-samples,  and  shooting  down 
large  samples  are  obsolete  methods,  as  a  bunch  of  rich  ore  is 
capable  of  salting  the  whole  sample,  and  any  sample  that  is 
too  large  to  be  sealed  in  a  sack  and  properly  protected  against 
salting  is  a  source  of  danger.  Small  mill  runs  are  not  satisfactory 
on  any  but  very  high-grade  ores,  as  the  clean-up  will  depend 
largely  upon  whether  the  plates  are  scraped  clean  or  whether 
they  are  allowed  to  absorb  amalgam. 

The  more  uniform  the  ore  the  smaller  may  be  the  samples; 
where  the  ore  is  spotty,  the  samples  should  be  large,  as  is  also 
the  case  where  the  ore  is  loose  and  breaks  irregularly,  or  is 
alternately  hard  and  soft. 

In  cutting  a  sample  the  rich  spots  should  be  avoided  if  they  are 
few  in  number;  if  there  are  many  rich  spots,  the  groove  should 
include  everything  along  its  line. 

It  is  a  very  difficult  matter  to  sample  correctly  a  spotty  ore. 
In  the  case  of  an  ore  that  consists  of  barren  or  nearly  barren  quartz 
carrying  free  gold,  the  average  cannot  well  be  determined  by 
sampling;  a  majority  of  the  samples  of  such  an  ore  will  be  very 
low,  or  blanks,  and  a  few  will  be  very  high;  the  average  obtained 
will  be  more  a  matter  of  luck  than  a  basis  for  an  accurate  estimate; 
a  mill  run  is  the  solution  of  this  problem. 

Salting. — The  best  safeguard  against  salting  is  to  decide  upon 
a  plan  for  safeguarding  samples  and  never  to  make  any  exception 


18  EXAMINATION  OF  PROSPECTS 

in  carrying  it  out,  whatever  the  circumstances;  if  an  engineer 
trusts  his  judgment  as  to  whether  he  is  in  safe  company,  his 
judgment  is  almost  certain  to  be  at  fault  at  some  time  during  his 
career,  but  if  he  always  maintains  the  same  vigilance  he  will 
never  be  salted.  Salting  is  the  result  of  carelessness,  and  is 
inexcusable. 

The  inclusion  of  waste  samples  is  generally  recommended  as  a 
safeguard,  but  like  much  good  advice,  is  rarely  carried  out. 

The  writer  insists  that  no  one  except  assistants  whose  integrity 
is  known  to  him  shall  approach  a  sample  until  it  is  placed  in  a 
new,  clean  sack,  and  sealed,  with  the  top  turned  over  and  tied 
down  to  prevent  the  working  in  of  fine  particles  at  the  mouth  of 
the  sack.  As  a  check  upon  a  series  of  samples  so  taken,  it  is  well 
to  save  a  portion  of  the  fines  from  rejected  quarterings  and  to  pan 
them;  if  a  black  greasy  scum  appears,  it  is  evidence  of  tellurides; 
if  a  string  of  colors  appears,  its  origin  should  be  ascertained. 

After  being  sacked  the  samples  should  be  locked  in  a  mail  sack, 
preferably  made  of  leather,  or  in  a  trunk.  If  the  sacks  used  are 
clean,  a  syringe  cannot  be  used  without  detection  through  the 
stain  left  on  the  inside  of  the  sack. 

It  is  much  better  to  offend  the  vendors  by  the  precautions 
taken  than  to  run  any  chances  of  being  salted;  those  with  honest 
intent  seldom  take  offense  at  such  precautions. 

The  Shipment  of  Samples. — Before  shipping  samples  it  is  best 
if  possible  to  grind  them  to  a  degree  of  fineness  such  that  the 
assay er  is  sure  to  take  a  representative  portion  for  assay;  this  is 
hard  work,  but  it  should  not  be  left  to  the  assistant  of  the  assayer 
employed,  who  may  not  give  proper  care  to  this  important  work. 

In  shipping  samples  it  is  well  to  direct  the  assayer  to  reserve 
the  pulp  in  case  an  umpire  assay  should  be  required,  which  will 
indicate  to  him  that  he  is  assaying  against  another  man  and  will 
so  induce  accuracy.  A  duplicate  set  of  samples  should  always 
be  kept  by  the  engineer.  With  low-grade  ores  it  is  best  to  in- 
struct the  assayer  to  make  crucible  assays  on  two  assay-ton  lots. 
Samples  should  be  packed  for  shipment  in  boxes  in  preference  to 
sacks. 


MINING  EXAMINATIONS  19 

It  should  be  unnecessary  to  state  that  local  assayers  should  be 
viewed  with  suspicion,  and  that  the  man  who  is  to  run  the  samples 
should  be  as  thoroughly  known  to  the  engineer  as  any  assistant 
whom  he  may  entrust  with  the  sampling.  In  large  examinations 
an  assayer  is  usually  included  in  the  staff. 

Resampling. — It  is  usually  advisable  to  resample  personally  a 
certain  proportion  of  the  cuts  as  a  check  upon  assistants,  as  well 
as  to  check  high  assays. 

High  Assays. — The  treatment  accorded  abnormally  high  assays 
will  vary  with  each  property  examined  and  with  every  engineer. 
The  usual  procedure  is  to  reassay  the  sample  one  or  more  times, 
to  determine  if  the  high  result  is  due  to  a  rich  speck  in  the  pulp 
taken  for  assay.  If  the  sample  as  a  whole  is  found  to  be  high, 
the  cut  should  be  resampled.  If  this  result  checks  the  first,  some 
engineers  recommend  resampling  halfway  between  the  high 
sample  and  the  adjacent  samples,  and  using  the  average  of  these 
results  in  place  of  the  high  result;  others  advocate  the  omission 
of  the  high  assay,  using  in  its  place  the  average  of  the  other 
assays  from  the  same  exposure. 

The  most  reasonable  basis  upon  which  to  consider  a  high  assay 
is  in  the  light  that  it  is  due  to  the  average  grade  of  ore  plus  an 
extra  amount  of  the  valuable  mineral,  and  to  substitute  for  it 
the  average  of  the  higher  samples  from  the  same  exposure  the 
results  of  which  have  been  accepted. 

The  Calculation  of  Results. — The  foot-ounce  method  is  the  one 
generally  adopted  in  the  calculation  of  ore  reserves.  Several 
elaborate  and  complicated  methods  have  been  put  forward  by 
various  engineers,  but  it  seems  probable  that  the  results  of  calcu- 
lation by  the  foot-ounce  method  are  as  accurate  as  the  results  of 
the  samples  themselves.  In  this  method  the  length  of  each  sam- 
ple is  multiplied  by  its  assay  value;  the  products  from  all  the 
samples  in  the  block  under  consideration  are  added,  and  this 
total  divided  by  the  sum  of  the  lengths,  the  quotient  being  the 
average  value. 

All  calculations  should  be  made  in  dollars  for  gold,  in  ounces 
for  silver,  and  in  percentages  for  other  metals,  and  these  values 


20  EXAMINATION  OF  PROSPECTS 

should  not  be  translated  into  dollars  per  ton  until  the  final  result 
for  a  given  block  is  obtained,  when  the  market  price  used  for  the 
various  metals  should  be  set  down  also. 

In  calculating  tonnages  the  specific  gravity  of  the  ore  should 
be  carefully  determined;  it  is  not  unusual  for  engineers  to  assume 
an  average  specific  gravity  for  the  ore,  a  procedure  that  is  likely 
to  lead  to  serious  error;  this  is  apparent  if  it  is  remembered  that 
the  percentage  error  in  the  ore  reserves  is  directly  proportional 
to  the  percentage  error  in  the  guess  at  the  specific  gravity. 
With  ores  composed  largely  of  heavy  sulphides,  the  specific 
gravity  may  be  determined  by  estimation,  which  requires  that 
the  percentage  of  iron  be  determined  -in  addition  to  the  other 
base  metals;  this  method  will  yield  accurate  results  and  should 
be  used  where  the  specific  gravity  of  the  ore  varies  greatly,  the 
calculation  being  applied  to  the  individual  sample  results.  The 
best  method  in  most  cases  is  to  determine  the  specific  gravity 
directly,  weighing  in  air  and  afterward  in  water  several  batches 
of  material  from  different  parts  of  the  ore-shoot  under  investiga- 
tion. The  method  of  packing  a  box  with  ore  and  determining  the 
weight  of  a  known  volume  of  broken  ore,  and  then  introducing 
a  factor  to  represent  the  relative  volumes  of  broken  ore  and  ore 
in  place,  is -open  to  objection;  the  factor  introduced  is  a  guess, 
and  the  result  depends  upon  the  ratio  of  voids  to  ore  and  there- 
fore upon  the  tightness  with  which  the  ore  is  packed  into  the  box. 

The  determination  of  the  specific  gravity  of  a  porous  ore  is  a 
difficult  matter;  it  may  best  be  accomplished  by  weighing  in  air 
and  determining  the  volume  of  the  pieces  weighed  by  overflow  of 
a  vessel  full  of  water.  If  this  is  done  rapidly  the  result  is  correct, 
but  does  not  take  into  account  vugs  or  open  spaces,  which  must 
be  allowed  for. 

Stoping  Width. — The  relation  between  the  width  of  a  vein  and 
the  stoping  width  necessary  to  extract  the  ore,  should  receive 
careful  attention  in  the  examination  of  narrow  veins.  The 
minimum  stoping  width  for  machine  drills  is  usually  from  4  to  5 
ft. ;  the  new  air-hammer  drills  require  less.  For  hand  drilling, 
30  in.,  or  sometimes  even  less,  is  required. 


MINING  EXAMINATIONS  21 

The  sloping  width  and  the  amount  of  waste  broken  with  the  ore 
varies  with  the  relative  hardness  of  the  ore  and  wall  rocks,  and 
also  according  to  the  firmness  of  the  hanging  wall,  which,  if 
loose,  will  contribute  waste  by  caving.  In  some  veins  the  waste 
may  be  shot  down  first  and  the  ore  broken  down  clean  afterward. 

In  the  consideration  of  narrow  veins  the  average  amount  and 
grade  of  ore  must  be  dertemined  as  it  stands,  and  the  values,  if 
any,  that  are  carried  by  the  wall  rock;  then,  upon  the  assumption 
of  a  stoping  width,  the  average  value  of  the  broken  ore  may  be 
calculated.  This  average  should  be  further  corrected  by  the 
amount  and  value  of  the  waste  that  may  economically  be  sorted 
out,  the  final  result  being  the  grade  that  may  be  expected  for  the 
ore  to  be  hoisted  or  shipped.  These  figures,  in  the  case  of  an 
operating  mine,  may  be  checked  by  records  of  past  production, 
where  the  volumes  of  material  stoped  can  be  determined,  or 
where  the  amount  and  grade  of  the  waste  sorted  out  is  known. 

Hand  Picking. — The  results  indicated  by  sampling  are  almost 
always  diminished  by  slabs  of  wall  rock  that  unavoidably  become 
mixed  with  the  ore.  In  deposits  that  carry  heavy  sulphides 
unevenly  disseminated  through  the  gangue,  and  in  ores  through 
which  the  values  are  irregularly  distributed  in  such  a  manner 
that  the  richer  portions  are  distinguishable  to  the  unaided  eye, 
the  question  of  hand  picking  is  of  prime  importance. 

Hand  picking  is  a  very  effective  process,  where  properly 
arranged  for  and  carried  out  on  clean  ore  from  which  the  fine 
material  has  been  screened.  Not  only  is  the  waste  sorted  out  in 
hand  picking,  but  clean  mineral  is  saved  as  a  high-grade  product. 
The  process  yields  two  clean  products,  finished  ore  and  clean 
waste,  in  one  operation,  and  thereby  saves  not  only  mill  capacity, 
and  the  cost  of  treating  the  waste,  but  also  greatly  reduces  the 
metallurgical  losses  that  the  clean  mineral  would  otherwise  suffer. 
An  inexpensive  installation  to  permit  efficient  sorting  will  often 
result  in  the  recovery  of  a  payable  product  from  an  ore  that  could 
not  otherwise  be  considered  an  asset. 

Metallurgical  Losses. — The  question  of  metallurgical  losses  is 
as  important  as  that  of  the  average  grade  of  the  ore;  in  all  cases 


22  EXAMINATION  OF  PROSPECTS 

it  should  receive  careful  attention,  and  in  formal  examinations 
an  exhaustive  series  of  tests  may  be  necessary.  The  mineralog- 
ical  character  of  the  ore  is  the  criterion  in  the  field  work,  unless 
laboratory  tests  can  be  made.  Before  a  plant  is  installed  or  a 
developed  property  purchased,  however,  large  scale  tests  are 
advisable. 

The  Cost  of  Mining  and  Treatment. — The  cost  of  mining  and 
treatment  is  a  factor  as  important  as  grade  of  the  ore,  and  is 
probably  more  subject  to  the  persojial  equation  than  any  other 
branch  of  an  examining  engineer's  work:  individual  judgment 
based  on  experience  is  the  final  guide  in  an  estimate  of  costs. 
This  subject,  which  applies  rather  to  the  examination  of  developed 
mines  than  to  prospects,  is  most  thoroughly  gone  into  in  Mr.  J.  R. 
Finlay's  book  on  "The  Cost  of  Mining." 

The  results  of  past  operations  must  be  considered  in  the  light 
of  future  probabilities,  and  it  should  be  borne  in  mind  that  mining 
costs  and  selling  prices  of  metals  vary  greatly  with  time,  and,  in 
some  cases,  with  the  seasons. 

The  larger  the  property  and  the  more  complete  its  development 
the  easier  becomes  the  problem  of  estimating  costs.  Those 
properties  that  most  nearly  approach  the  character  of  a  manu- 
facturing enterprise  are  the  easiest  with  which  the  engineer  has 
to  deal. 

In  the  examination  of  an  isolated  prospect  having  no  ore 
reserves,  a  detailed  discussion  of  probable  costs  is  out  of  place, 
as  the  costs  will  vary  according  to  the  tonnage  developed.  A 
tentative  estimate  based  on  experience  is  the  best  that  may  be 
offered  in  such  cases. 

The  regularity  of  a  deposit  and  its  absolute  size  are  important 
factors  in  the  cost  of  mining;  the  angle  of -inclination  of  the  de- 
posit with  the  horizon  is  important,  also,  as  determining  whether 
the  ore  when  broken  in  the  stopes  will  run  or  whether  it  will  have 
to  be  shoveled. 

All  the  other  attributes  of  a  prospect  must  be  considered  in  the 
light  of  its  situation  with  respect  to  transportation;  excessive 
distances,  or  a  rugged  topography  without  roads  or  trails,  may 


MINING  EXAMINATIONS  23 

render  valueless  a  property  that  would  be  valuable  if  better 
situated. 

Water  is  a  necessity  for  camp  use  and  for  metallurgical  plants, 
and  if  there  is  no  visible  source  of  water  near  a  prospect,  the 
question  at  once  becomes  grave.  Some  gold  mines  in  the  Altar 
District  of  Sonora,  and  in  many  other  desert  regions,  are  commer- 
cially impossible  on  account  of  scanty  water  supply.  Water  in 
old  workings  should  be  investigated,  and  its  source  determined— 
whether  it  is  due  to  seepage,  or  whether  it  comes  from  an  under- 
ground channel.  The  amount  of  flow  of  water  is,  of  course,  of 
vital  importance,  and  the  question  as  to  whether  the  flow  comes 
from  an  underground  reservoir  which  may  ultimately  be  drained, 
or  from  a  regularly  flowing  channel,  is  important.  Examples 
are  numerous  where  large  flows  of  water  have  rendered  impossible 
the  mining  of  otherwise  valuable  ore-bodies. 

Climate  and  altitude  are  not  usually  controlling  factors  in  the 
United  States,  but  become  such  in  the  high  latitudes,  or  in  the 
fever  belts  in  the  tropics.  The  high  altitude  has  been  a  great 
detriment  to  the  development  of  certain  districts  in  southwestern 
Colorado. 

Wages  and  supply  of  labor  are  factors  for  careful  consideration ; 
fuel  and  motive  power  also  are  factors  of  prime  importance. 

It  is  often  advanced  that  a  company,  through  large  expenditure 
for  equipment,  may  greatly  better  the  working  costs  of  a  property 
that  has  been  operating  on  a  small  scale;  this  is  rarely  true  with 
small  high-grade  mines,  where  the  small  owner  can  secure  working 
costs  comparing  favorably  with  anything  that  a  large  company 
can  accomplish.  This  is  the  reason  that  small  high-grade  mines 
are  usually  poor  purchases;  their  owners  demand  all  that  they  are, 
or  may  become,  worth.  Good  examples  of  this  condition  are 
offered  by  many  mines  in  Mexico  that  produce  ore  as  cheaply 
with  primitive  methods  as  could  a  large  company  with  expensive 
installations. 

The  cost  of  equipment  must  finally  be  charged  off  against  the 
tonnage  mined,  and  for  this  reason  large  low-grade  properties  are 
likely  to  offer  the  best  opportunities  to  the  average  investor. 


24  EXAMINATION  OF  PROSPECTS 

In  determining  the  probable  cost  of  mining,  the  development 
cost  must  not  be  forgotten;  the  footage  of  shafts,  drifts,  raises, 
and  so  forth  must  be  considered  in  connection  with  the  amount 
of  ore  developed,  and  the  probable  cost  of  future  development 
must  be  included  in  the  estimate  of  the  mining  costs. 

Finally,  the  purchase  price  is  a  charge  of  which  each  ton  must 
bear  its  share.  These  factors  are  often  forgotten  in  America, 
where  mining  risks  are  too  often  accepted  in  a  gambling  spirit. 

The  Estimation  of  Ore  Reserves. — Most  engineers  will  agree 
fairly  well  in  regard  to  the  quantities  of  developed  ore  and  of 
probable  ore  in  any  mine,  but  great  differences  are  to  be  expected 
in  estimates  of  possible  ore,  which,  of  necessity,  are  forced  predic- 
tions of  the  future  based  upon  insufficient  data.  A  reasonable 
method  by  which  to  evade  such  predictions  is  suggested  by  Mr. 
J.  H.  Curie' *  it  is  to  purchase  mines  on  the  basis  of  the  ore  reserves 
plus  a  royalty  for  all  the  ore  afterward  discovered. 

The  starting-point  in  a  consideration  of  the  probable  depth  to 
which  an  ore-shoot  will  persist  is  the  determination  whether  the 
ore  is  of  primary  or  of  secondary  origin;  if  primary,  there  is  no 
genetic  reason  why  the  shoots  should  not  continue  to  great  depths; 
if  secondary,  little  or  no  tonnage  may  be  allowed  below  the  lowest 
workings,  unless  the  mineralogical  character  of  the  ore  is  such 
that  it  seems  reasonably  certain  that  the  workings  are  still  in  the 
upper  part  of  the  zone  of  enrichment.  Secondary  ore-shoots,  in 
general,  are  greater  in  horizontal  than  in  vertical  extent;  primary 
ore-shoots,  on  the  contrary,  are  commonly  greater  in  vertical 
than  in  horizontal  extent;  the  relation  between  the  horizontal 
and  vertical  dimensions  of  exposed  ore-shoots  should  be  con- 
sidered in  making  estimates  of  the  depth  to  which  they  probably 
continue. 

If  the  ore  exposed  is  the  result  of  geological  conditions  unlikely 
to  be  duplicated  in  depth,  the  end  of  the  mine  is  probably  in 
sight.  « 

It  is  certainly  true  that  the  tendency  of  primary  as  well  as  of 
secondary  ore-shoots  is  to  pinch  out  or  to  become  low  in  grade 
1  "The  Gold  Mines  of  the  World, "  p.  46. 


MINING  EXAMINATIONS  25 

with  increasing  depth;  many  primary  ores,  however,  probably 
continue  to  depths  below  the  limit  of  mining. 

A  factor  of  safety  for  the  engineer's  personal  reputation  has  no 
place  in  a  mining  report,  except  in  the  margin  of  profit  that  he 
deems  necessary  to  make  the  enterprise  attractive. 


CHAPTER  II    ^ 
STRUCTURAL  GEOLOGY 

Ore-deposits  are  commonly  divided  into  two  classes,  syngenetic 
and  epigenetic,  according  to  whether  the  ore  was  deposited  to- 
gether with  the  enclosing  rock  or  was  introduced  Rafter  its 
deposition  or  solidification.  Epigenetic  deposits,  which  are  by 
far  the  more  important  class,  owe  their  formation  to  the  channels 
that  permitted  the  ingress  of  their  metals,  and  both  classes  are 
subject  to  great  modification  by  post-mineral  changes  in  the 
containing  rocks.  A  discussion  of  the  structural  features  of 
rocks,  therefore,  necessarily  precedes  any  consideration  of  ore- 
deposits  or  processes  of  ore  deposition. 

Stratification. — The  arrangement  of  sediments  in  parallel  and 
approximately  horizontal  layers  is  called  stratification,  and  is  an 
original  property  of  sedimentary  rocks.  Successive  strata  of  the 
same  sedimentary  bed  commonly  differ  from  each  other  in  some 
minor  characteristic,  such  as  texture  or  color,  which  results  in  a 
bedded  or  stratified  appearance. 

Cleavage. — The  subjection  of  a  rock  to  pressure  develops  within 
it  parallel  planes  of  weakness,  called  cleavage  planes,  along  which 
the  rock  breaks  into  relatively  regular  slabs  or  blocks;  the  direc- 
tion of  these  cleavage  planes,  which  bear  no  relation  to  stratifica- 
tion, is  determined  by  the  direction  of  the  pressure  that  produced 
them.  Where  there  are  more  than  one  set  of  cleavage  planes,  one 
of  them  is  likely  to  be  more  prominent  than  the  others.  In  sedi- 
mentary rocks,  the  cleavage  may  coincide  with  the  stratification, 
but  more  commonly  cuts  across  it.  Cleavage  has  been  defined1 
as  the  "  capacity  present  in  some  rocks  to  break  in  some  directions 
more  easily  than  in  others."  Cleavage,  therefore,  does  not  imply 
the  existence  of  subdivisions,  but  rather  the  tendency  to  subdivide 
along  certain  planes. 

1  Van  Hise. 

26 


STRUCTURAL  GEOLOGY  27 

Fissility. — Fissility  is  a  "  structure  in  rocks  by  virtue  of  which 
they  are  already  separated  into  parallel  laminae."1  Fissility 
may  be  regarded  as  a 'development  of  the  property  of  cleavage, 
and  is  commonly  expressed  along  clo'sely  set  parallel  planes. 

Schistose  Structure. — The  long-continued  stresses  of  regional 
metamorphism  with  accompanying  recrystallization  and  rock 
flowage  develop  a  banded  structure  in  both  sedimentary  and 
igneous  rocks;  the  various  minerals  contained  by  the  rocks  are 
arranged  with  their  longer  axes  parallel  and  form  planes  of  weak- 
ness to  fracture  that  differ  from  cleavage  planes  and  that  ^are 
independent  of  any  original  stratification.  A  rock  that  has 
suffered  this  change  is  said  to  have  a  schistose  structure.  Schists 
commonly  exhibit  cleavage  that  has  no  relation  to  their  schistos- 
ity,  and  occasionally,  also,  traces  of  the  original  stratification  are 
visible. 

Gneissic  Structure. — A  coarsely  schistose  structure  is  known 
as  a  gneissic  structure.  In  gneisses  the  individual  bands  are 
more  prominently  developed  than  in  schists,  and  the  ease  of 
fracture  along  these  bands  is  relatively  less. 

Joints. — Planes  of  division  through  rocks  that  are  the  result 
of -stresses  insufficient  to  produce  more  than  microscopic  move- 
ment are  called  joints.  Fragments  of  broken  rock,  both  sedi- 
mentary and  igneous,  are  commonly  bounded  in  part  by  plane 
surfaces,  which  are  joint  planes.  Joints  are  of  several  kinds, 
according  to  the  nature  of  the  stresses  that  produced  them;  they 
vary  in  expression  from  cleavage,  or  incipient  jointing,  to  the  well- 
developed  planes  that  bound  the  prismatic  columns  of  certain 
basalts.  Joint  planes  frequently  preserve  their  general  direc- 
tions over  long  distances,  and  the  angles  between  different  joint 
systems  are  likely  to  be  constant  throughout  large  rock  masses. 

Where  the  joint  planes  are  closely  spaced  and  where  the  load  of 
overlying  rocks  is  light,  they  afford  channels  for  the  circulation 
of  solutions  and  frequently  become  mineralized. 

During  the  contraction  that  results  from  cooling  and  solidifica- 
tion, igneous  rocks  separate  into  masses  of  roughly  polygonal 

1  Van  Hise. 


28 


EXAMINATION  OF  PROSPECTS 


section  bounded  by  tension  or  contraction  joints.  Such  joints 
are  best  developed  in  flows,  which  upon  solidification  divide  into 
regular  prismatic  columns. 

Sedimentary  rocks  upon  drying  not  infrequently  suffer  con- 
traction, which  fmds  expression  in  similar,  but  usually  less  well- 
defined,  planes  of  division.  Fractures  due  to  contraction  are 
the  result  of  internal  strains  in  a  rock  mass,  and  do  not  pass 
outside  of  it  into  other  rocks  for  any  important  distance. 


FIG.  3. — Slates  on  Elk  Creek,  Idaho,  showing  joint  planes  at  nearly  right 
angles  to  the  bedding.     After  Ransome. 

The  outer  members  of  folded  strata  undergo  tensile  stress 
during  folding,  which  occasionally  results  in  the  formation  of 
tension  joints  that  follow  a  radial  arrangement  inward  from  the 
arc  of  the  fold. 

Sheeting. — Parallel  planes  of  fracture,  developed  by  compres- 
sive  stresses,  of  relatively  great  continuity  as  compared  with  j  oints, 
but  of  only  incipient  displacement,  are  called  sheeting  planes. 
Closely  set  and  well-developed  sheeting  planes  often  afford 


STRUCTURAL  GEOLOGY 


29 


FIG.  4. — Devil's  Tower,  Wyoming,  showing  columnar  jointing  of  igneous 
rock.     After  Darton. 


30 


EXAMINATION  OF  PROSPECTS 


channels  for  the  circulation  of  solutions,  and,  when  mineralized? 
form  sheeted  lodes.  Not  infrequently  systems  of  sheeting  planes 
occur  in  pairs,  parallel  in  strike,  but  intersecting  in  dip,  such 


w 


Foot 


Wall 


of 


Hanging 


Wall 


FIG.  5. — Ideal  section  of  the  Ferris-Haggerty  ore-body,  Encampment, 
Wyoming,  showing  convexity  and  the  mineralization  of  tension  joints. 
After  Spencer. 


Q  Qn  Q. 

FIG.  6. — Section  of  folded  strata  near  Negaunee,  Michigan,  gn,  Gneiss; 
g,  granite;  q,  quartzite;  s,  clay  slate;  e,  iron-bearing  Negaunee  strata;  d, 
diorite  and  diabase.  After  Van  Rise  and  Bay  ley. 


Feet 
Above  Sea  Level 

-8500 


,'''    ,''"','- •«>     ;     /,'Coppe'r  B^dsL__ 


•7500 
6500 
5500 
4500 

,.3500 

A  A 

FIG.  7. — Partly  eroded  ayticline,    Montpelier,  Idaho.     After  Gale. 

fracturing  being  the  typical  result  of  compressive  or  torsional 
strains;  these  interdependent  systems  of  sheeting  planes  are 
known  as  conjugate  systems. 


STRUCTURAL  GEOLOGY 


31 


Folds. — A  fold  is  a  bend  in  a  rock  mass  caused  by  compressive 
stress  of  insufficient  intensity  to  produce  a  fault.  A  fold  is  called 
a  syncline  if  its  bend  is  concave  above,  and  an  anticline  if  its  bend 
is  concave  below.  A  dome  is  an  anticline  whose  length  is  zero, 


FIG.  8. — Section  through  the  ore-deposit  at  Meggen,  Germany,  showing 
a  synclinal  trough,  k,  pyrite;  s,  barite;  Is  and  6s,  slates;  kn,  limestone. 
After  Hundt. 

and  is  best  described  by  the  usual  meaning  of  its  name.  A  basin 
is  the  synclinal  equivalent  of  a  dome.  Folds  are  commonly 
persistent  in  strike,  and  domes  or  basins  are  of  relatively  rare 
occurrence.  A  monocline  may  be  described  as  a  half-fold,  as 


FIG.  9. — Monocline,    near   Gallup,    New   Mexico.     After   Howett. 

where  strata  assume  a  terrace-like  position,  being  horizontal  at 
different  elevations  either  side  of  an  inclined  connecting  part. 
The  rock  forming  the  convex  or  outer  portion  of  a  fold  is,  like 
the  lower  portion  of  a  beam  subjected  to  load,  under  tensile 
stress,  which  frequently  results  in  a  series  of  fractures  that  may 


32 


EXAMINATION  OF  PROSPECTS 


afford  channels  for  the  circulation  of  solutions  and  so  become  loci 

of  ore  deposition. 

The   Development  of  Faults   from  Folds. — Where   a  fold   is 

formed  by  compressive  stress 
beyond  the  capacity  of  the 
rock  to  withstand,  a  fault  is 
developed  along  the  axis,  or 
the  plane  bisecting  the  angle 
between  the  component  limbs 
of  the  fold.  Folds  not  infre- 
quently pass  into  faults  along 
their  strike,  and  in  a  region 
that  has  been  subjected  to 


FIG.  10. — Specimen  of  folded  slate, 
Black  Hills,  South  Dakota,  showing 
a  fissure  developed  along  the  axis  of 
the  fold.  After  Irving. 


FIG.  11. — A  flexure.     After 
Beck. 


both  folding  and  faulting  these  expressions  of  stress  are  likely 
to  be  parallel. 
Fractures. — As  generally  used,  the  term  fracture  denotes  a 


STRUCTURAL  GEOLOGY 


33 


break  in  a  rock  mass  in  importance  intermediate  between  a 
joint  and  a  fault,  as  these  latter  terms  are  generally  used.1 

Flexures. — A  flexure  is  a  sharp  bend  in  a  series  of  strata  or 
in  a  rock  mass,  without  the  development  of  a  continuous  fracture, 
the  result  being  a  displacement  similar  to  that  of  a  monocline. 
Flexures  readily  pass  into  faults. 

Faults. — A  fault  is  a  fracture  through  a  rock  mass  the  opposite 
walls  of  which  have  moved  past  each  other,  the  word  indicating 
lack  of  correspondence  between  opposite  walls. 


FIG.  12. — Normal  faults:  a  recent  fault  near  the  surface,  and  the  same 
with  the  hanging  dropped  down.     After  Beck. 

The  strike  of  a  fault  is  the  direction  of  a  horizontal  line  within 
the  plane  of  the  fault ;  the  dip  of  a  fault  is  the  angle  between  the 
plane  of  the  fault  and  a  horizontal  plane. 

The  distance  measured  on  the  plane  of  a  fault  between  the  new 
positions  of  two  points  that  were  originally  opposite  is  called  the 
total  displacement,  which  distance  may,  for  purposes  of  calcula- 
tion, be  considered  as  the  hypothenuse  of  a  right  triangle,  whose 
sides  represent  the  horizontal  and  the  vertical  movements  of 
which  it  is  the  resultant. 

The  terms  throw  and  heave  as  commonly  used  refer  respec- 
tively to  the  vertical  and  horizontal  distances  between  the 
new  positions  of  originally  opposite  points  in  a  fault  the  move- 
ment along  which  was  directly  down  the  dip;  by  offset  is  com- 

1  J.  E.  Spurr,  "Geology  Applied  to  Mining,"  p.  177. 
3 


34 


EXAMINATION  OF  PROSPECTS 


monly  meant  the  horizontal  distance  (perpendicular  to  the  strike) 
between  the  two  portions  of  a  faulted  vein,  without  regard  to  the 
direction  or  amount  of  total  displacement. 

In  a  great  majority  of  faults  the  total  displacement  is  not 
directly  down  the  dip,  but  is  the  result  of  both  horizontal  and 
vertical  movements. 

A  normal  fault  is  a  fault  along  which  the  hanging  wall  has 
moved  downward  on  the  foot-wall,  and  is  the  natural  result  of  a 
drawing  apart  of  the  two  rock  masses,  the  least  supported  mass 
slipping  downward  on  the  mass  having  the  larger  base. 1  Normal 
faults  are  commonly  the  effect  of  gravity,  and  due  to  tension, 


FIG.   13. — Reverse  fault,  Coeur  d'Alenes,  Idaho.     After  Ransome. 

although  they  sometimes  result  from  compressive  stresses. 
Faults  due  to  tension  commonly  find  expression  in  a  simple  frac- 
ture and  are  not  accompanied  by  sheeting. 

A  reverse  fault  is  one  along  which  the  hanging  wall  has  moved 
upward  on  the  foot-wall,  which  is  commonly  considered  as  having 
been  forced  under  the  overlying  mass  by  compression.  Such 
faults  are  less  likely  to  be  simple  fractures  than  normal  faults, 
and  are  frequently  accompanied  by  parallel  sheeting,  or  by  a 
folding  of  the  strata  or  rock  masses  that  form  their  walls. 

Faults  due  to  torsional  stresses  are  commonly  accompanied  by 
differential  movement,  the  displacement  over  one  part  of  the 
fault  being  greater  than  over  other  parts,  resulting  in  a  tilting  of 
the  faulted  block.  Such  faults  frequently  follow  curved  lines  of 
strike,  and  are  often  branching. 

1  J.  F.  Kemp,  "Ore  Deposits,"  p.  21. 


STRUCTURAL  GEOLOGY 


35 


Faults  are  sometimes  referred  to  as  strike  faults,  or  as  dip 
faults,  according  to  their  direction  as  compared  to  the  strike  or 
dip  of  enclosing  strata  or  associated  veins. 

A  series  of  approximately  parallel  faults  of  similar  class  and 
displacement  are  called  step  faults;  if  of  opposite  displacement, 
they  are  called  compensating  faults. 

Two  faults,  or  two  systems  of  faults,  parallel  in  strike  but  inter- 


Granite 


Gxay-Porphyry         Quartzite  Limestone  Quartzite 

FIG.  14.  —  Section  perpendicular  to  the  strike  of  a  series  of  step  faults 
at  Leadville,  Colorado:  The  total  displacement  of  a  fault  that  extends  for 
a  long  distance  through  the  region  is  here  distributed  among  several  parallel 
faults.  After  Emmons  and  Irving. 

secting  in  dip,  are  occasionally  formed  by  the  same  compressive 
or  torsional  stresses,  and  are  known  as  conjugate  faults  or  fault 
systems. 

Faults  formed  at  slight  depth  below  the  surface  are  likely  to 
be  less  regular  and  less  persistent  than  deep-seated  faults.  The 
occurrence  of  many  small,  irregular  fractures  that  become  less  in 
number  and  more  regular  in  direction  as  depth  is  gained,  is  indi- 


36  EXAMINATION  OF  PROSPECTS 

cative  of  faulting  under  light  load  and  at  slight  depth;  a  further 
indication  of  shallow  formation  is  the  presence  of  friction- breccia 
along  a  fault  rather  than  the  pasty  gouge  that  is  characteristic  of 
deep-seated  movements  under  heavy  loads  of  overlying  rocks.  The 
upward  branching  of  a  fault,  or  a  marked  change  in  dip,  is  a  sign 
of  shallow  dislocation.  Surface  detrital  matter  is  occasionally 
found  in  a  fault  filling,  conclusively  proving  a  shallow  depth  at 
the  time  of  faulting. 


Scale  of  Feet 
10 15  20  25 


FIG.  15. — Horizontal  sketch  plan  of  a  part  of  the  Mizpah  vein,  Tonopah, 
Nevada,  showing  probable  compensating  faulting.     After  Spurr. 

Exploration  for  the  Continuation  of  a  Faulted  Ore -body. — Where 
an  ore-body  has  been  cut  off  by  a  fault  the  discovery  of  its  con- 
tinuation beyond  the  fault  is  a  problem  of  the  greatest  impor- 
tance. Numerous  rules  have  been  formulated  to  aid  in  a  search 
for  the  continuation  of  a  faulted  ore-body,  but  it  is  safe  to  say 
that  the  great  variety  and  complexity  of  the  results  of  faulting 
render  such  rules  valueless  in  most  cases. 

Where  faulting  dislocates  a  non-tabular  ore-body  that  is 
contained  in  a  homogeneous  rock,  the  problem  is  insoluble, 
unless  the  fault  carries  a  drag  or  trail  of  ore  mixed  with  the 
fault  filling. 

The  direction  of  the  striations  on  the  walls  of  a  fault,  in  simple 
cases,  indicates  the  direction  of  movement,  the  deeper  ends  of  the 
furroughs  being  pointed  in  the  opposite  direction  to  the  move- 
ment of  the  opposite  walls.  Occasionally  an  ore-body  is  de- 


STRUCTURAL  GEOLOGY  37 

formed  where  faulted,  the  deformation  pointing  in  the  direction 
of  displacement  and  passing  into  a  drag  or  trail  through  the  fault 
filling. 

Where  a  tabular  ore-body  or  vein  is  faulted,  the  problem  is 
likely  to  be  simpler,  as  the  recovery  of  any  portion  of  the  vein 
beyond  the  fault  will  lead  to  the  discovery  of  the  continuation  of 
the  ore-shoot.  Where  the  enclosing  rock  is  composed  of  a  series 
of  strata  a  knowledge  of  the  succession  of  the  strata  is  of  the 
greatest  assistance  in  the  calculation. 

The  cases  are  rare  where  complete  data  for  the  calculation  of 
the  total  displacement  are  available,  as,  for  instance,  where  a 
fault  cuts  a  dike  that  itself  cuts  a  series  of  known  strata,  or  an 
older  fault,  or  any  combination  of  intersections  that  permits  the 
recognition  of  a  point  rather  than  of  a  plane  on  both  sides  of  the 
fault. 

In  an  endeavor  to  locate  the  continuation  of  a  faulted  ore-body 
the  striations  or  slickensides  should  be  studied,  and  also  the  fault 
filling  for  possible  fragments  of  ore,  or  of  country  rock  other  than 
that  of  the  immediate  walls  of  the  fault;  a  survey,  map,  and 
sections  should  be  made  showing  the  dip  and  strike  of  both  the 
fault  and  of  the  ore-body,  if  of  tabular  form,  and  of  every  known 
stratum  of  the  containing  rock,  igneous  contact  or  dike,  older 
fault  or  vein,  or  any  distinguishing  mark  that  may  be  recognized 
on  both  sides  of  the  fault.  The  application  of  descriptive  geome- 
try, or  trigonometry,  to  this  data  will  result  in  all  the  information 
regarding  the  displacement  that  the  situation  renders  possible; 
complicated  formulae,  like  rules  of  thumb,  are  of  no  assistance  in 
the  solution  of  such  problems. 

It  is  seldom  that  the  direction  and  amount  of  total  displace- 
ment may  be  measured  directly,  more  frequently  it  is  susceptible 
of  calculation  from  related  functions,  and  frequently  it  is  not 
determinable  until  further  exploration  discloses  the  necessary 
data. 

A  comparison  with  known  faults  in  the  same  district  is  fre- 
quently of  great  help  in  the  consideration  of  an  unknown  displace- 
ment, and  an  apparently  complicated  fault  system  once  worked 


38  EXAMINATION  OF  PROSPECTS 

out  may  become  a  relatively  simple  problem  when  again  met 
» with  underground. 

Post -mineral  Fissures. — A  line  of  weakness  once  established  is 
likely  to  continue  as  such  through  long  periods  of  time,  the 
original  gouge  seam  or  zone  of  attrition  material  being  preserved 
and  offering  a  plane  of  easy  relief  to  subsequent  stresses;  where  a 
fissure  has  been  completely  healed  by  mineralization,  the  result- 
ing vein  of  brittle  quartz  or  other  minerals  is  likely  to  be  less 
resistant  to  fracture  than  the  tougher  enclosing  rock.  It  is 
common,  therefore,  to  find  evidence  of  post-mineral  movement 
along  veins. 

Post-mineral  movement  is  likely  to  crush  the  ore  and  to  mix 
it  with  waste  to  such  an  extent  that  its  value  is  materially  reduced 
and  also  to  render  the  country  rock  loose  and  likely  to  cave 
during  mining  operations;  not  infrequently,  post-mineral  move- 
ment so  complicates  the  structure  as  to  render  exploration 
difficult,  or  unremunerative. 

A  favorable  effect  of  post-mineral  fissuring,  either  along  or 
across  a  vein  or  deposit,  is  in  permitting  the  access  of  surface 
waters,  which  are  thereby  given  opportunity  to  form  secondary 
enrichments,  as  will  be  taken  up  in  a  later  chapter. 

A  usual  effect  of  post-mineral  movement  along  a  vein  is  the 
formation  of  false  walls,  or  fissures  carrying  gouge,  that  cut  the 
vein  obliquely;  these  mask  the  part  of  the  vein  behind  them,  and 
also  hide  the  junctions  of  branch  veins.  False  walls  are  frequently 
the  cause  of  losing  a  vein  in  development,  the  tendency  being  to 
follow  their  well-defined  fissures  and  to  leave  the  vein  to  one  side. 
Where  such  movements  are  known  to  have  taken  place,  frequent 
cross-cutting  may  be  necessary  to  make  sure  that  the  whole  of 
the  vein  has  been  exposed. 

Pre -mineral  Fissures.1 — Gouge-filled  fissures  appear  to  be 
unfavorable  to  ore  deposition,  probably  chiefly  because  the 
pasty  gouge  does  not  permit  the  ready  passage  of  mineralizing 
solutions,  and  also  because  the  continual  movement  along  such 

1  This  subject  is  fully  taken  up  by  Mr.  F.  L.  Ransome  in  P.  P.  62,  U.  S. 
G.  S.;  p.  120. 


STRUCTURAL  GEOLOGY  39 

fissures  tends  to  close  any  minute  channels  that  are  permitted  to 
form.     Such  fissures  once  formed  probably  persist  as  lines  of1 
weakness  and  movement  for  long  periods,  and  the  occurrence  of 
a   gouge-filled   fissure  in  connection  with  an  ore-deposit  is  na 
proof  that  it  is  of  later  formation  than  the  ore. 

It  is  frequently  seen  in  mines  that  such  fissures  act  as  efficient 
dams,  the  passage  of  solutions  through  them  being  as  difficult 
as  the  circulation  of  solutions  along  them;  they  maybe  considered, 
therefore,  as  having  frequently  determined  the  limits  of  ore 
deposition  through  the  impounding  of  mineralizing  solutions. 

The  fact  that  an  ore-body  or  vein  is  cut  off  by  a  gouge-filled 
fissure  is  no  proof  that  a  fault  has  displaced  the  ore-body  or  vein, 
the  continuation  of  which  may  never  have  existed  beyond  the 
fissure.  It  is  often  difficult  to  prove  whether  such  a  fissure  is 
of  later  formation,  and  has  faulted  a  vein,  or  whether  it  is  older 
than  the  vein,  which  ends  upon  reaching  it.  The  proof  of  a 
fault  is,  of  course,  the  continuation  of  the  vein  beyond  it;  this, 
however,  is  the  object  of  the  search.  That  such  a  fissure  is  a 
post-mineral  fault  may  be  indicated  by  the  faulting  of  associated 
beds  or  dikes,  or  by  a  drag  or  trail  of  ore  through  the  fault  filling. 

If  such  a  gouge-filled  fissure  is  older  than  and  limits  the  ore, 
this  may  be  indicated  in  a  filled  fissure,  by  a  closing  of  lines  of 
crustification  against  the  fault,  equivalent  bands  being  connected, 
or  by  a  change  in  the  vein  filling  upon  approaching  the  fault,  or 
by  a  branching. or  widening  of  the  vein  upon  approaching  the 
fault. 

A  gouge-filled  fissure  older  than  the  ore  may  through  recent 
movement  exhibit  the  characteristics  of  a  fault  that  has  dis- 
placed the  vein,  though  in  reality  limiting  it. 

The  Expression  of  Faults  in  Topography. — The  more  recent  a 
fault,  and  the  greater  the  difference  in  resistance  to  erosion 
between  the  rocks  of  its  walls,  the  more  likely  is  it  to  find  expres- 
sion in  the  topography;  a  fault  of  great  displacement  that  finds 
no  expression  in  the  topography  is  probably  not  of  recent  origin. 
Faults  may  in  some  cases  be  recognized  at  the  surface  by  features 
other  than  the  lack  of  correspondence  between  their  walls,  such 


40  EXAMINATION  OF  PROSPECTS 

as  a  fault  scarp,  a  saddle  in  a  ridge,  or  the  course  of  a  canyon,  or 
through  the  outcropping  of  a  fault  breccia  that  has  become 
silicified  and  so  rendered  resistant  to  erosion.  In  most  cases, 
however,  erosion  is  the  dominant  feature  in  controlling  the  topog- 
raphy, and  structural  features  are  rarely  represented  at  the 
surface,  except  through  relative  resistance  to  erosion. 

The  Importance  of  Aereal  Geology. — The  surface  affords  a 
complete  section  of  the  geological  features  of  any  district,  and  in 
all  but  the  simplest  occurrences  a  geological  map'  of  the  surface 
and  a  few  vertical  sections  are  invaluable  guides  in  the  examina- 
tion or  exploration  of  any  district.  .  All  significant  outcrops  of 
rocks,  dikes,  beds,  veins,  faults,  shear  zones  and  sf)  forth  should 
be  determined  and  their  strikes,  dips,  and  elevations  recorded  on 
a  large  scale  map,  from  which  the  data  may  be  referred  for  study 
to  a  horizontal  plane.  Contour  maps  are  useful  in  such  work, 
but  in  most  cases  the  expense  of  contouring  is  not  justified,  and 
the  elevation  of  significant  readings  will  be  found  to  be  sufficient. 

A  horizontal  section  thus  prepared,  with,  perhaps,  several 
vertical  sections,  will  indicate  through  lack  of  correspondence  of 
strata  or  other  criteria,  the  existence  and  location  of  faults  that 
otherwise  might  not  be  suspected.  If  the  surface  observations 
are  abundant,  they  often  afford  data  from  which  the  displace- 
ments of  faults  may  be  calculated. 

A  geological  map  of  the  surface  results  in  dividing  a  district 
into  fault  blocks,  conditions  within  each  of  which  may  be  con- 
sidered as  constant,  but  which  must  be  expected  to  change  upon 
passing  from  one  fault  block  to  another.  The  importance  of 
dividing  a  district  into  fault  blocks  will  be  appreciated  on  consid- 
ering that  a  prospect  may  be  situated  within  a  few  hundred  feet 
of  an  important  mine,  but  actually  separated  from  it,  perhaps, 
by  a  fault  thousands  of  feet  in  throw,  or  in  a  formation  that  is 
apparently  similar,  although  actually  unrelated,  to  the  ore-bear- 
ing rock. 

Exploration  and  Development. — In  the  opening  of  any  property 
two  distinct  objects  should  be  kept  in  mind — the  development  of 
known  ore-bodies,  and  the  exploration  for  further  deposits. 


STRUCTURAL  GEOLOGY 


41 


Where  payable  ore  has  been  found,  it  should  be  followed  with 
a  view  to  the  development  of  ore  and  also  to  gain  knowledge  in 
regard  to  its  occurrence,  each  ore-shoot  being  opened  individually 
without  the  testing  of  theories.  In  general,  the  time  to  go  slowest 
in  following  an  ore-body  is  when  it  shows  signs  of  giving  out; 


FIG.  16. — Map  showing  the  fault  blocks  at  Tonopah,  Nevada;  the  payable 
veins  occur  in  the  earlier  andesite  and  are  most  prominent  in  the  fault 
block  bounded  by  the  Burro,  Mizpah,  Stone  Cabin,  and  Gold  Hill  faults. 
After  Spurr. 

every  detail  should  then  be  studied  and  recorded  before  the  rock 
has  been  dirtied  by  further  blasting,  as  frequently  small  stringers 
or  gouge-filled  fissures  will  be  found  to  lead  from  one  ore-body  to 
another. 

In  the  exploration  for  further  ore-shoots,  however,  the  major 
features  of  the  distribution  of  ore  should  be  borne  in  mind,  and 


42  EXAMINATION  OF  PROSPECTS 

the  general  trend  of  the  deposit  should  be  cross-cut  thoroughly. 
Exploration  should  be  confined  at  first  to  cross-cutting  the  known 
ore-bearing  horizon  or  zone  of  mineralization — perhaps  a  sedi- 
mentary bed  favorable  to  the  deposition  of  ore,  a  zone  of  crushing 
or  brecciation,  or  a  certain  horizon  known  to  be  the  most  favor- 
able for  secondary  enrichments.  Within  these  broad  zones  or 
horizons  considered  favorable  to  the  existence  of  ore,  the  minor 
features,  such  as  stringers  of  ore,  low-grade  ore,  sheeted  zones 
and  so  forth,  may  be  considered  important.  Upon  cross-cutting 
the  apparent  trend  of  the  deposit  at  the  horizon  considered  most 
favorable  to  the  existence  of  ore,  the  most  promising  stringer  or 
seam  should  be  followed  along  its  strike  for  an  appropriate  dis- 
tance, where  another  cross-cut  is  in  order. 

The  plan  to  be  followed  demands  detailed  study  in  each  indi- 
vidual case,  and  a  state  of  mind  is  necessary  that  is  receptive  of 
new  impressions  as  the  work  progresses.  Preliminary  explora- 
tion is  frequently  entrusted  to  a  practical  miner,  when  as  a  matter 
of  fact,  such  work  constitutes  perhaps  the  most  important  field  of 
the  trained  geologist. 


CHAPTER  III 
STRUCTURAL  FEATURES  OF  ORE -DEPOSITS 

Veins,  Lodes  and  Ledges. — Many  definitions  have  been  advanced 
and  many  limitations  advocated  in  the  use  of  these  terms.  The 
following  definitions  appear  to  follow  the  best  usage. 


F-F' 


.Dacite 


Dacite 


FIG.  17. — Plan  and  section  of  the  Combination  ledge,  Goldfield,  Nevada, 
showing  the  irregular  occurrence  of  the  ore  within  the  ledge.     After  Ran- 


Fissure  Veins. — A  fissure  vein  is  a  mineral  mass  tabular  in  form 
as  a  whole,  though  often  irregular  in  detail,  filling  or  accompany- 
ing a  fracture  or  series  of  closely  set  and  intimately  related  par- 
allel fractures  in  the  enclosing  rock,  the  mineral  mass  having  been 

43 


44 


EXAMINATION  OF  PROSPECTS 


formed  later  than  both  the  country  rock  and  the  fracture,  either 
through  the  filling  of  open  spaces  along  the  fracture  or  through 
chemical  alteration  of  the  adjoining  rock.1 

Lodes. — A  lode  is  a  zone  of  fissuring  that  contains  roughly 
parallel  mineral  masses  of  the  general  type  of  fissure  veins,  usu- 
ally connected  by  cross  veins  and  mineralized  breccias  to  such 
a  degree  that  over  certain  portions  the  whole  width  constitutes  a 
single  ore-body. 


ll  \ 


FIG.  18. — Gash  veins,  Upper  Mississippi  Valley;  the  black  shading  indicates 
galena.     After  Kemp. 

A  ledge  is  an  irregular  mass  of  altered  rock,  containing  ore 
bodies,  the  alteration  of  which  is  due  to  and  characteristic  of  the 
action  of  mineralizing  solutions.2 

Gash  Veins. — A  gash  vein  is  a  vein  of  superficial  character, 
widest  near  the  surface  and  narrowing  to  extinction  in  depth. 
Gash  veins  are  usually  the  results  of  solution  and  deposition 

1  Waldemar  Lindgren. 

2  F.  L.  Ransome. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


45 


along  joints  or  small  fissures  by  surface  waters,  and  are  of  second- 
ary origin.  The  term  has  been  less  correctly  applied  to  lenticu- 
lar deposits  that,  prominent  at  the  surface,  die  out  in  a  similar 
manner  in  depth.  The  usual  occurrence  of  gash  veins  is  in  sedi- 
mentary rocks.  The  typically  short  length  of  this  type  of  vein 
should  not  lead  to  an  expectation  of  continuity  in  depth,  even  if 
the  origin  is  not  suspected  from  study  of  the  outcrop. 

Bed  Veins. — A  bed  vein  is  a  vein  that  follows  a  bedding  plane  of 
an  enclosing  sedimentary  rock,  less  frequently  a  plane  between 


FIG.  19. — Section  of  a  bed  vein  (copper)  in  the  Snowstorm  mine,  Coeur 
d'Alenes,  Idaho.     AJter  Ransome. 

layers  of  volcanic  rocks.  Bed  veins  are  commonly  thought  to 
be  less  persistent  than  veins  that  cut  across  the  strata  of  enclosing 
rocks;  many  cases  are  known,  however,  where  bed  veins  are  both 
persistent  and  contain  important  ore-shoots.  Blanket  vein  is 
often  used  as  a  synonym  for  bed  vein,  but  actually  refers  to  a 
horizontal  or  nearly  horizontal  position  only. 

A  bed  vein  in  unaltered  rocks  is  sometimes  distinguished  with 
difficulty  from  a  stratum  whose  mineralization  was  contempora- 
neous with  its  own  deposition.  In  the  examination  of  an  unal- 
tered bed,  an  opinion  may  usually  be  based  upon  the  closeness 
with  which  the  mineralization  follows  the  minute  bedding  planes 


46 


EXAMINATION  OF  PROSPECTS 


as  well  as  the  larger  bed  divisions;  if  the  mineralization  follows  the 
larger  bedding  features  only,  and  does  not  penetrate  the  relatively 
solid  intervening  strata,  it  is  probably  later  in  origin  than  the  bed 
itself.  Fragments  of  country  rock  or  the  presence  of  cross  or 
branching  stringers  are  definite  signs  of  a  later  origin.  Further- 
more, a  mineralization  that  is  not  Relatively  continuous  through 
a  certain  stratum,  but  is  transferred  to  strata  above  and  below 
without  corresponding  lateral  extent,  is  later  than  the  containing 
bed. 


FIG.  20. — Diagrammatic  section  of  the  Tonopah  Extension  vein,  Tonopah, 
Nevada,  showing  a  compound  vein.  A,  Altered  wall  rock;  B,  typical  white 
vein  of  the  earlier  andesite  period,  containing  silver  sulphides  and  carrying 
values  of  several  hundred  dollars  per  ton;  C,  black,  jaspery  quartz  of  later 
introduction  than  the  original  vein  of  which  it  carries  fragments — the  value 
of  the  black  quartz  and  fragments  varies  from  twenty  to  thirty  dollars 
per  ton.  After  Spurr. 

IN  THE  GOEUR  D'ALENES,  IDAHO/  a  majority  of  the  auriferous 
quartz  veins  are  of  the  bed-vein  type.  These  veins  in  some 
places  occur  singly,  but  more  commonly  they  occur  in  groups, 
individual  veins  being  separated  by  a  few  inches  or  a  few  feet  of 
slaty  country  rock.  An  overlapping  arrangement  is  common, 
one  vein  gradually  pinching  out  while  a  parallel  vein  between 

1  F.  L.  Ransome,  P.  P.  62,  U.  S.  G.  S.,  p.  141. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS  47 

adjacent  beds  becomes  correspondingly  thicker.  Although 
certain  individual  veins  .persist  for  hundreds  of  feet  without 
cutting  across  the  planes  of  stratification,  such  crossings  may  be 
observed  here  and  there,  and  cross-cutting  stringers  of  quartz 
that  link  neighboring  veins  are  numerous. 

Compound  Veins. — A  compound  vein  is  a  vein  that  has  been 
reopened  after  mineralization,  and  again  mineralized,  containing, 


FIG.  21. — Open  pit  of  the  Treadwell  mine,  Douglas  Island,  Alaska,  show- 
ing an  albite-diorite  dike  that  has  been  brecciated  and  impregnated  with 
ore.  After  Spencer. 

perhaps,  two  sets  of  ores.  A  frequent  case  is  that  of  a  vein 
composed  chiefly  of  barren  or  low-grade  filling  that  has  received 
its  valuable  mineral  upon  being  reopened. 

Contact  Veins. — A  contact  between  two  rocks  occasionally 
offers  a  line  of  weakness  to  fracturing  that  later  mineralization 
may  transform  into  a  contact  vein.  Such  a  vein  is  rarely  regular 
over  long  distances,  unless  its  position  along  the  contact  be 


48  EXAMINATION  OF  PROSPECTS 

accidental,  due  to  faulting.  Veins  of  secondary  minerals  are 
likely  to  form  along  a  contact  where  one  wall  offers  an  impervious 
barrier  to  migration  of  solutions  and  so  induces  precipitation 
along  its  course  through  impounding. 

Veins  Along  Dikes. — Dikes  of  intrusive  rocks,  after  solidifica- 
tion, are  likely  to  remain  lines  of  weakness  along  which  new 
fractures  readily  form;  veins  frequently  follow  dikes,  either  along 
one  wall  or  through  the  mass  of  the  dike  itself. 


FIG.  22. — Section  of  the  Howard  lode,  Cripple  Creek,  Colorado,  showing 
a  sheeted  zone;  the  medial  portion  of  the  lode  is  closely  sheeted,  the  sheet- 
ing planes  becoming  gradually  farther  apart  in  the  foot-  and  hanging-walls. 
After  Lindgren  and  Ransome. 

AT  CRIPPLE  CREEK,  COLORADO,  many  of  the  veins  follow  phono- 
lite  or  basic  dikes. 

Lodes  Along  Sheeted  Zones. — A  sheeted  zone  is  a  series  of 
closely  set  parallel  sheeting  planes;  they  frequently  afford  chan- 
nels for  mineralizing  solutions  and  so  become  mineral  lodes.  Such 
lodes  are  commonly  not  as  persistent  as  fissure  veins,  but  mineral 
deposits  may  follow  them  for  long  distances,  the  mineralization 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS  49 

being  found  sometimes  in  one  and  sometimes  in  another  set  of 
sheeting  planes,  following,  perhaps,  an  overlapping  arrangement. 
Sheeting  planes  are  commonly  parallel  to  the  main  fissures,  and 
lodes  are  frequently  made  up  of  a  mineralized  fissure  with  one  or 
more  mineralized  sheeting  planes  nearby.  There  is  usually  no 
evidence  of  movement  along  sheeting  planes. 

Where  there  has  been  much  replacement  along  a  lode  the  sheet- 
ing planes  are  likely  to  be  obscured,  and  the  term  is  usually 
applied,  therefore,  to  lodes  that  carry  their  minerals  in  a  scanty 
gangue.  A  typical  lode  along  a  sheeted  zone  is  made  up  of 
narrow  parallel  veinlets  separated  by  slabs  of  country  rock. 

Frequent  cross-cutting  is  advisable  in  the  exploration  of  a 
mineralized  sheeted  zone. 

AT  CRIPPLE  CREEK,  COLORADO/  sheeted  zones  consist  of  a  num- 
ber of  narrow,  approximately  parallel,  fissures,  which  collectively 
form  lodes  ranging  from  a  few  inches  up  to  50  or  60  ft.,  or  rarely, 
100  ft.,  in  width.  Within  such  wide  belts  of  fracture,  however, 
two  or  more  zones  of  concentrated  fissuring  may  usually  be 
distinguished  that  lie  close  enough  together  to  be  mined  as  a 
whole.  In  other  words,  the  very  wide  sheeted  zones  are  com- 
pound sheeted  zones.  As  a  rule,  the  fissures  are  mere  cracks, 
showing  no  brecciation,  slickensiding,  or  other  evidence  of  tan- 
gential movement  of  the  walls;  there  are  some  notable  exceptions 
to  this  statement,  but  the  movement  of  one  wall  past  the  other 
has  probably  in  few  instances  exceeded  1  or  2  ft.  In  general,the 
sheeted  zones  are  from  2  to  10  ft.  in  width.  Among  the  numerous 
and  important  lodes  that  properly  come  under  the  designation  of 
sheeted  zone,  several  structural  varieties  may  be  distinguished, 
which  are  sometimes  exhibited  in  different  parts  of  the  same  lode. 
A  common  form  is  that  characterized  by  the  presence  of  two  main 
parallel  fissures,  usually  3  or  4  ft.  apart,  accompanied  by  less 
regular  and  less  persistent  fractures  in  the  intervening  and  adja- 
cent rock.  In  another  common  type  of  sheeted  zone  the  parallel 
fissures  are  more  numerous,  and  are  spaced  with  some  regularity. 
There  is  usually  a  medial  portion  of  the  lode  that  ranges  from  a 

1  Waldemar  Lindgren,  P.  P.  54,  U.  S.  G.  S.,  p.  160. 


50 


EXAMINATION  OF  PROSPECTS 


few  inches  to  a  foot  or  two  in  width,  within  which  the  rock  is 
divided  into  a  large  number  of  very  thin  plates  by  fissures  often 
less  than  an  inch  apart;  this  band  of  intense  sheeting  is  accom- 
panied on  both  sides  by  parallel  fissures  that  are  spaced  farther 
and  farther  apart,  so  that  the  sheeted  zone  as  a  whole  merges 
gradually  into  the  country  rock.  There  are  a  large  number  of 
sheeted  zones  in  breccia  and  in  granite  that  are  composed  of 
many  parallel  or  nearly  parallel  fissures,  but  that  differ  from  the 
type  just  described  in  the  absence  of  a  well-defined  medial  zone 
and  in  the  rather  less  regular  character  of  the  fractures. 


4  Level 


Dives 
1 


Feet 

0   100  200  300  400  500  60C 

lit    iii    I 


FIG.  23. — Sketch  plan  of  the  underground  workings  of  the  North  Star 
mine,  Silverton,  Colorado,  showing  the  irregularity  of  mineralization  of  a 
stringer  lode.  After  Ransome. 

Stringer  Lodes. — A  lode  in  which  the  mineralization  has  fol- 
lowed a  net-work  of  irregular  curving  fissures  that  have  no 
general  parallelism  among  themselves,  but  follow  a  general  trend 
similar  to  that  of  a  sheeted  zone  is  known  as  a  stringer  lode. 
This  type  of  lode  rarely  has  definite  walls,  and  the  ore  may  be 
encountered  at  any  point  within  the  general  zone  of  fissuring;  as 
with  sheeted  zones,  frequent  cross-cutting  is  necessary  in  the 
exploration  of  this  type  of  deposit. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


51 


Fault  Lodes. — A  zone  of  faulting  in  which  mineralization  has 
taken  place  irregularly  through  the  crushed  and  comminuted 
fault  material  is  known  as  a  fault  lode.  This  type  of  deposit, 
where  the  values  are  irregularly  distributed  either  with  or  without 
a  scanty  gangue,  is  not  uncommon  in  the  areas  of  old  schists  in 
the  desert  region  of  the  Southwest.  These  lodes  are  difficult  to 
follow,  and  their  exploration  in  advance  of  actual  mining  is 
usually  unprofitable. 

The  Mineralization  of  Joints. — A  strongly  jointed  rock  offers 


FIG.  24. — Mineralization  of  joints,  Monte  Cristo,  Washington.      After 

Spurr. 


many  lines  of  weakness  to  fissuring  stresses,  and  not  infrequently 
a  fissure  that  is  well-defined  in  depth  is  dissipated  through  the 
joint  planes  upon  nearing  the  surface.  A  mineralization  that 
has  taken  place  at  relatively  shallow  depth,  therefore,  not  infre- 
quently follows  the  joint  planes  and  is  distributed  among  them 
as  a  system  of  reticulated  veinlets.  Such  disseminations  in 
depth  are  likely  to  coalesce  into  well-defined  lodes.  A  system  of 
mineralized  joint  planes  is  commonly  the  result  of  surface 
agencies;  less  frequently  it  is  the  result  of  a  primary  mineral!- 


52 


EXAMINATION  OF  PROSPECTS 


zation    at    shallow    depth   in  a  region  where  erosion  has  been 
slight. 

Breccia  Lodes. — A  zone  of  shattering  in  which  the  mineraliza- 
tion has  cemented  or  replaced  the  brecciated  mass  of  angular 
fragments  and  comminuted  material  is  known  as  a  breccia  lode. 
This  type  of  deposit  is  commonly  irregular,  the  mineralization 
varying  with  the  amount  and  degree  of  brecciation.  In  extreme 
cases  the  brecciation  may  have  been  out  of  proportion  to  the 


FIG.  25. — Mineralization    of    joints,     Monte    Cristo,     Washington.     After 

Spurr. 

quantity  of  mineralizing  solutions,  and  have  dissipated  these 
solutions  through  so  large  a  mass  of  rock  that  the  resulting  ore 
body  is  very  low  in  grade. 

Shear  Zones. — A  zone  of  incipient  fissuring  or  shearing  that 
has  been  mineralized  by  impregnating  solutions,  commonly  by 
replacement,  and  to  a  less  extent,  perhaps,  by  the  filling  of  open 
spaces,  is  known  as  a  shear  zone.  The  passage  of  solutions 
through  such  zones  is  probably  slow,  and  ample  time  is  afforded 
for  mineralization  by  replacement.  While  genetically  a  shear 
zone  may  be  of  any  size,  the  term  as  generally  used  is  applied  to 
large,  low-grade  deposits. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


53 


Stockworks. — An  area  through  which  numerous  veins  traverse 
the  rock  in  all  directions,  forming  a  net-work  through  mutual 
intersection,  is  known  as  a  stockwork.  In  typical  cases  the 
individual  veins  are  small  and  are  considered  collectively  as  a 
deposit. 

Stocks. — A  stock  is  a  deposit  of  irregular  form  due  chiefly  to 
replacement  of  the  containing  rock.1 


Surface 


Surface 


Yankee  Girl  Mine. 
Section 

O.B.  and  Y.G.  Chimney 

Ore  — 


7th  Level 


FIG.  26. — Ore  chimneys  in  the  Yankee  Girl  mine,  Red  Mountain,  Colorado. 

After  Schwartz. 

Pipes  or  Chimneys. — Certain  deposits  take  the  form  of  a  pipe 
or  chimney,  and  have  marked  vertical  continuity,  with  very 
subordinate  horizontal  dimensions.  It  is  difficult  to  conceive  of 
a  fissure  whose  only  important  dimension  is  approximately 
vertical,  and  these  deposits  were  probably  formed  at  the  inter- 
section of  fissures  that  throughout  the  remainder  of  their  lengths 

1  Beck- Weed,  "Nature  of  Ore  Deposits,"  p.  51. 


54 


EXAMINATION  OF  PROSPECTS 


did  not  permit  the  passage  of  mineralizing  solutions,  and  so  failed 
to  be  emphasized  in  connection  with  the  main  deposit;  other 
deposits  of  this  form  appear  to  be  due  to  the  mineralization  of 
fumarolic  vents.  Certain  pipes  in  limestone  appear  to  have 
been  formed  by  solution,  probably  along  a  fracture,  or  intersec- 
tion of  fractures:  these  are  likely  to  be  quite  irregular  in  form. 

AT  THE  BASSICK  MINE,  OUSTER  COUNTY,  COLORADO,  an  ore-de- 
posit that  varies  from  20  to  100  ft.  in  horizontal  diameter  has  been 


FIG.  27. — Plan  of  the  productive  lodes  west  of  Silver  Lake,  Colorado, 
showing  a  branching  vein  system;  the  lodes  farthest  south  are  an  example 
of  an  overlapping  system.  After  Ransome. 

followed  to  a  depth  of  over  800  ft.  This  chimney  occurs  in  a 
volcanic  neck  and  the  ores  have  formed  in  concentric  layers 
about  boulders  of  the  volcanic  agglomerate. 

Branching  Veins. — Many  veins  send  off  branches  into  their 
hanging-  or  foot-walls,  usually  into  the  former;  beyond  such 
branches  veins  frequently  lose  in  either  width  or  value.  A  vein 
that  branches  along  its  strike  may  or  may  not  unite  farther  on, 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


55 


but,  in  general,  the  branching  of  a  vein  is  likely  to  indicate  the 
dying  out  of  its  fissure.  If  possible,  therefore,  development  work 
should  follow  the  direction  in  which  the  branches  are  converging. 
While  no  definite  rule  may  be  formulated,  branch  veins  are  likely 


FIG.  28. — General  sketch  of  the  lode  system  of  the  Upper  Harz,  Germany, 
showing  a  linked-vein  system.     After  Beck. 

to  diminish  in  both  size  and  value  with  distance  from  the  main 
vein,  which  usually  formed  the  main  channel  of  mineralizing 
solutions,  and  of  which  the  branch  veins  were  only  lateral  and 
dependant  channels. 

Linked  Veins. — A  system  of  veins  that  branch  and  reunite 


56 


EXAMINATION  OF  PROSPECTS 

200  Level 


Scale  of.Feet 

FIG.  29. — Section  of  the  Tomboy  and  Iron  lodes,  Silverton,  Colorado.   After 
Herron  and  Ransome. 


FIG.  30. — Intersecting,  or  conjugate,  joint  systems,  mineralized,  Encamp- 
ment, Wyoming.     After  Spencer. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS  57 

without  crossing  one  another  is  called  a  linked-vein  system. 
Where  such  a  relation  exists  between  veins,  the  arrangement 
appears  likely  to  be  persistent. 

Conjugate  Veins. — Fractures  produced  by  compressive  stress 
through  homogeneous  rocks  are  likely  to  form  systems  parallel 
in  strike  but  opposite  in  dip,  known  as  conjugate  fractures; 
where  mineralized  these  become  conjugate  veins,  of  which  one 
dip  is  usually  emphasized  while  the  other  is  subordinate,  or 
exists  as  a  simple  fissure  only.  Conjugate  joint  systems  are  of 
common  occurrence.  The  intersection  of  two  systems  of  intense 
jointing  appear  to  be  favorable  loci  for  ore  deposition. 

Overlapping  Veins. — It  often  happens  that  the  stresses  pro- 
ducing fractures  do  not  find  expression  in  a  single  persistent 
fissure,  but  in  a  number  of  more  or  less  closely  spaced  parallel 
fissures  that  partly  overlap  one  another,  forming  a  step-like 
system.  Such  a  system  is  known  as  a  system  of  overlapping 
fissures.  Overlapping  veins  are  probably  the  result  of  fissuring 
stresses  that  were  exerted  along  a  line  other  than  the  line  of  least 
resistance  of  the  rock  mass;  while  the  lines  of  least  resistance 
of  the  rock  were  followed  over  relatively  short  distances, 
the  final  expression  of  the  stress  was  along  the  average  strike  of 
the  individual  fissures  taken  as  a  whole,  and  parallel  to  the  simple 
fissure  that  would  have  formed  had  the  rock  been  homogeneous. 

Many  vein  systems  follow  overlapping  fractures,  and  this 
arrangement  probably  gave  rise  to  the  miner's  rule  to  cross-cut 
"into  the  hanging"  or  "into  the  foot"  upon  losing  a  vein,  the 
overlapping  being  either  persistently  to  the  right,  or  persistently 
to  the  left,  in  most  cases.  An  overlapping  vein  system  is  also 
spoken  of  as  having  an  arrangement  en  echelon.  Blind  veins, 
or  veins  that  do  not  outcrop,  frequently  belong  to  an  overlapping 
vein  system,  one  of  whose  members  outcropped  and  so  led  to 
the  discovery  of  those  that  did  not. 

Systems  of  Related  Veins. — A  careful  plotting  of  the  veins  of 
any  mining  district  will  often  indicate  a  certain  relationship 
between  the  payable  fissures  as  regards  strike,  dip,  or  distribu- 
tion. A  clustering  of  veins  about  a  particular  intrusive  mass  is 


58 


EXAMINATION  OF  PROSPECTS 


frequently  observed,  and  the  important  veins  in  many  districts 
maintain  a  fairly  parallel  alignment.  Mr.  J.  M.  Boutwell,1 
has  determined  that  at  Bingham,  Utah,  over  84  per  cent,  of  the 
payable  fissures  strike  between  N.5°  E.  and  N.  43°  E.,  and  that 


FIG.  31. — Diagram  showing  the  trends  of  the  barren  fissures,  the  lean, 
and  the  payable  veins  and  lodes  of  the  Bingham  District,  Utah.  After 
Boutwell. 

over  80  per  cent,  of  them  dip  between  40°  and  90°  NW.     In  this 
district  all  fissures,  barren,  lean  and  payable,  taken  together, 
strike  about  equally  in  all  directions,  but  the  northeast  fissures 
1  P.  P.  38,  U.  S.  G.  S.,  p.  363. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


59 


appear  to  have  been  those  that  afforded  channels  for  solutions  at 
the  time  of  mineralization.  A  striking  instance  of  the  parallelism 
of  pay  fissures  is  exhibited  in  the  Bald  Mountain  District,  South 
Dakota. 

A  roughly  radial  distribution  of  veins  about  a  common  center 
has  been  noted  in  certain  districts,  and  while  well  established  in 


FIG.  32. — Map  of  the  principal  ore-shoots  of  the  Bald  Mountain  area, 
Black  Hills,  South  Dakota,  showing  the  marked  parallelism  of  the  payable 
fissures.  After  Irving. 

these  districts,  it  is  of  rare  occurrence.  Cripple  Creek,  Colorado, 
affords  perhaps  the  best  known  example.  The  veins  at  Cerro  de 
Potosi,  S.  A.,  are  said  to  follow  a  roughly  radial  distribution. 
While  the  recognition  of  a  definite  vein  system  is  a  valuable 
guide  in  exploration,  the  vein  systems  that  have  been  cited  indi- 


60 


EXAMINATION  OF  PROSPECTS 


Lodes 


FIG.  33. — Plan  of  the  principal  veins  of  the  Cripple  Creek  District, 
Colorado,  showing  a  roughly  radial  distribution  about  a  common  center. 
After  Lindgren  and  Ransome. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS  61 

cate  rather  more  clear  relationships  than  commonly  obtain 
among  the  veins  of  most  districts. 

In  districts  where  the  veins  vary  greatly  in  strike  and  dip  it  is 
easy  to  construct  relationships  that  do  not  exist,  and  thus  arrive 
at  misleading  conclusions. 

The  Persistence  of  Veins  in  Depth. — There  is  a  distinct  relation- 
ship between  the  length  and  depth  of  veins;  in  general,  long, 
strong,  wide  veins  persist  in  depth  and  short,  non-continuous, 
irregular  and  weak  veins  die  out  at  no  great  distance  beneath 
the  surface,  perhaps  to  be  followed  by  similar  and  roughly 
parallel  veins  in  depth.  While  ore-bearing  fissures  are  com- 
monly not  important  as  faults,  there  is  a  relation  between  the 
amount  of  throw  and  the  length  arid  depth  of  a  fissure;  the 
greater  the  throw,  the  greater  are  likely  to  be  these  dimensions. 

In  the  consideration  of  individual  veins  that  have  pinched 
in  depth,  a  decision  will  rest  upon  the  behavior  of  the  vein  in  the 
parts  already  explored.  If  the  proved  length  of  the  vein  is  con- 
siderably greater  than  the  depth  attained,  and  if  the  displacement 
along  the  vein  is  known  to  have  been  more  than  slight,  the 
chances  are  good  that  the  fissure  will  persist  in  depth.  This  has 
long  been  recognized  by  miners,  who  consider  well-developed 
slickensides  an  indication  of  persistency  in  depth.  If  a  vein  has 
been  subject  to  local  pinches  above,  it  may  be  safely  assumed 
that  it  will  open  out  again  with  deeper  exploration.  In  general, 
it  may  be  said  that  the  behavior  of  a  vein  in  horizontal  exposures 
is  likely  to  be  roughly  duplicated  down  its  dip.  A  possible 
change  in  the  country  rock  should  be  borne  in  mind,  however,  as 
fissures  and  veins  are  likely  to  change  markedly  in  structure  upon 
passing  from  one  rock  into  another. 

A  vein  that  is  about  to  die  out  is  likely  to  split  into  several 
diverging  and  diminishing  stringers,  and  gradually  to  become 
lost  in  the  country  rock.  This  termination  is  more  often  noted 
in  horizontal  directions  than  in  the  dying  out  of  a  vein  in  depth, 
probably  for  the  reason  that  few  veins  are  followed  in  depth  to 
their  complete  extinction. 

The  probabilities  in  regard  to  the  ultimate  depth  of  fissures  and 


62  EXAMINATION  OF  PROSPECTS 

veins,  while  of  theoretical  interest,  have  little  or  no  practical 
significance.  The  depth  at  which  the  plasticity  of  rocks  under 
great  pressure  no  longer  permits  the  existence  of  openings  is  far 
below  the  depth  of  possible  mining  operations.1 

The  Relation  Between  Depth  and  the  Number  and  Character  of 
Veins. — In  most  mining  districts  veins  are  more  numerous  at 
and  near  the  surface  than  in  depth.  Veins  are  much  more  likely 
to  come  together  in  depth  than  to  divide  as  they  go  down,  and 
many  subordinate  and  weak  veins  die  out  altogether  within 
relatively  slight  depths  below  the  surface.  It  is  usual,  moreover, 
for  veins  to  become  more  regular  in  strike,  dip  and  width  in  depth 
than  they  are  nearer  the  surface,  although  they  are  commonly 
narrower  in  their  deeper  than  in  their  upper  parts.  It  appears 
probable  that  fissuring  stresses  are  limited  in  their  effect  at 
considerable  depth  to  a  few  fissures  or  to  a  single  fissure,  whereas 
near  the  surface  the  lesser  burden  of  superposed  rock  permits  a 
dissipation  of  the  stresses  among  a  number  of  more  irregular 
fractures. 

Mineral  Veins  Follow  Fissures  of  Small  Displacement. — Frac- 
tures through  rock  masses  may  be  said  to  vary  from  planes  of 
straining  and  incipient  fracture  to  faults  of  great  throw.  Min- 
eralization may  take  place  along  zones  of  strained  rocks,  and  while 
such  conditions  are  structurally  favorable  to  replacement,  the 
passage  of  solutions  must  be  slow  at  best.  The  other  extreme, 
that  of  faults  of  great  throw,  appears  to  be  unfavorable  to  the 
passage  of  solutions  and  mineralization. 

Fractures  through  rocks  are  rarely  plane  surfaces;  they  com- 
monly follow  curves  over  long  distances,  and  through  short 
distances  are  subject  to  many  irregularities  in  strike  and  dip.  The 
movement  of  one  wall  of  a  fissure  past  the  other  wall  produces 
much  finely  ground  material  known  as  gouge,  which  in  faults 
of  large  displacement  commonly  fills  the  fissure  tightly,  cement- 
ing the  rock  fragments  and  preventing  any  free  flow  of  solutions. 

1  Prof.  Van  Hise  gives  as  his  opinion  that  at  depths  of  from  10,000  to  12,000 
meters  the  weight  of  the  overlying  rocks  is  too  great  to  permit  the  hardest 
rocks  to  retain  their  form. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


63 


Faults  of  small  displacement  are,  in  general,  channels  favorable 
to  the  ready  passage  of  solutions.  The  irregularities  of  the  walls 
not  having  been  planed  off,  slight  movement  of  one  wall  past  the 
other  brings  the  concavities  opposite  one  another,  thus  forming 
open  spaces;  furthermore,  the  strained  rocks  bordering  such 
fissures  not  having  been  ground  up  into  gouge,  permit  the  solu- 
tions to  circulate  through  them  and  present  conditions  favorable 
to  replacement.  Most  payable  veins  follow  fissures  of  small  dis- 
placement, although  there  are  notable  exceptions  to  this  rule. 

The  Influence  of  Country  Rock  on  Vein  Structure. — A  fissuring 
stress  that  produces  an  efficient  circulation  channel  in  one  rock 


FIG.  34. — Scattering  of  the  Gottlob  Vein,  Freiberg,  Germany,  upon 
passing  from  gneiss  into  quartz-porphyry,  g,  Gneiss;  p,  quartz-porphyry; 
w,  vein.  After  Beck. 

may  be  too  great,  or  too  small,  to  produce  a  like  result  in  a  rock  of 
different  character.  The  size,  regularity  and  character  of 
fissures  vary  according  to  the  physical  properties  of  the  enclosing 
rocks,  such  as  homogeneity,  friability  or  plasticity. 

A  vein  that  traverses  more  than  one  country  rock,  therefore,  is 
likely  to  vary  greatly  in  structure  in  the  several  rocks.  Fissures 
of  small  displacement,  which  are  the  sort  commonly  followed  by 
mineral  veins,  may  be  strong  and  clean  cut  through  a  massive 
rock,  but  may  fade  out,  and  become  lost  in  a  plastic  rock,  where 
flowage  or  distortion  is  likely  to  take  up  the  movement  as  a  slight 
fold,  without  the  production  of  a  fracture. 


64 


EXAMINATION  OF  PROSPECTS 


Scale  of  Feet 
50  100 


150 


FIG.  35. — Horizontal  plan  of  the  vein  in  the  Maine  mine,  Georgetown, 
Colorado,  showing  thickening  of  the  vein  in  passing  from  gneiss  into  the 
harder  porphyry.  After  Spurr  and  Garrey. 


FIG.  36. — Horizontal  plan  of  a  part  of  the  Seven  Thirty  vein,  Georgetown, 
Colorado,  showing  diminution  in  the  size  of  the  vein  in  its  passage  from 
granite  into  porphyry.  After  Spurr  and  Garrey. 


STRUCTURAL  FEATURES  OF  ORE-DEPOSITS 


65 


A  vein  that  is  persistent  and  regular  in  a  homogeneous  rock 
upon  passing  into  a  brittle,  unyielding  rock,  or  into  a  rock  mass 
that  is  seamed  with  closely  spaced  joints,  is  likely  to  become 
dissipated,  and  its  mineralization  scattered  through  so  large  a 


a  b          c  d        b       c  b 

FIG.  37. — Diagrammatic  section  of  the  Sheba  ore-body,  near  Unionville 
Nevada,  showing  the  behavior  of  the  vein  in  different  rocks,  a,  Limestone; 
b,  tuff;  c,  porphyry;  d,  ore.  After  Ransome. 


Roof 


clay  selvae,e 

of  ore 


FIG.  38. — Behavior  of  vein  at    Neihart,  Montana,   upon  passing  from 
schist  into  amphibolite.     After  Weed, 

mass  that  the  resulting  deposit  is  of  too  low  grade  to  permit 
extraction. 

Veins  that  are  contained  in  a  large  mass  of  igneous  rock,  or 
other  rock  of  relatively  constant  character,  are  likely  to  be  more 

5 


66 


EXAMINATION  OF  PROSPECTS 


regular  and  more  persistent  than  veins  that  traverse  a  series  of 
bedded  sediments. 

Where  it  is  seen,  therefore,  that  a  vein  is  to  pass  from  the  rock 
through  which  it  has  been  followed  into  a  rock  of  different  charac- 
ter, decision  should  be  reserved  as  to  its  probable  continuity, 
unless  abundant  local  evidence  indicates  that  the  change  else- 
where in  the  district  is  without  effect,  or  is  favorable  in  character. 

The  behavior  of  a  fissure  at  a  contact  between  two  rocks  de- 
pends largely  upon  the  angle  at  which  it  meets,  the  contact. 


FIG.  39. — Sketch  of  a  vein  in  the  Gomer  mine,  Idaho  Springs,  Colorado, 
showing  the  deflection  of  the  vein  upon  meeting  a  dike.     After  Spurr. 

If  the  angle  is  acute,  the  fissure  may  be  deflected  along  the  con- 
tact for  some  distance,  and,  upon  crossing  it,  continue  with  a 
strike  parallel  to  its  original  course.  If  the  fissure  meets  the 
contact  at  nearly  right  angles,  it  is  likely  to  cross  it  without 
deflection,  or,  more  rarely,  to  cease  altogether. 

In  general,  shales,  schists  and  slates  are  more  likely  to  absorb 
stresses  without  the  production  of  definite  or  continuous  fissures, 
and  so  to  permit  a  dissemination  of  solutions  and  resultant 
mineralization,  than  are  rocks  of  non-fissile  structure.  Glassy 
rhyolites,  quartzites  and  other  rigid  and  brittle  rocks  are  likely 


;       STRUCTURAL  FEATURES  OF  ORE-DEPOSITS  67 

to  be  shattered  by  stress,  and  a  mineralization  that  is  regular  in 
another  rock  may  upon  encountering  them  become  disseminated 
through  a  large  brecciated  mass. 

Soft,  plastic  intrusives  are  likely  to  adjust  themselves  to 
stress  without  fracture,  and  lodes  upon  passing  into  such  rocks 
are  unlikely  to  continue  far  into  them;  in  this  connection  it  should 
be  borne  in  mind  that  the  softening  of  igneous  rocks  through  the 
metasomatic  action  of  mineralizing  solutions  takes  place  during 
mineralization,  and  is  not  a  factor  in  the  formation  of  the 
fissures;  such  alteration  indicates  rather  than  militates  against 
the  continuity  of  associated  fissures. 

AT  NEIHART,  MONTANA,1  a  series  of  steeply  dipping  metamor- 
phic  rocks,  consisting  of  feldspathic  gneiss,  softer  bands  of  more 
schistose  rocks,  and  occasional  tough  amphibolites,  is  cut  by  an 
irregular  intrusion  of  diorite,  and  all  in  turn  are  intruded  by  a 
rhyolite  porphyry.  Well-defined  fissure  veins  cross  all  these 
rocks,  meeting  the  metamorphic  strata  at  nearly  right  angles. 
The  veins  vary  somewhat  in  width  and  in  the  relative  abundance 
of  included  rock  fragments  upon  passing  from  one  belt  of  felds- 
pathic gneiss  to  another,  and  more  markedly  where  they  pass  into 
the  more  schistose  rocks,  but  the  change  is  complete  and  abrupt 
where  they  meet  the  amphibolites;  here  the  veins  commonly 
narrow  from  a  width  of  7  or  8  ft.  of  good  ore  to  a  foot  or  more  of 
barren  gahgue.  Upon  passing  from  the  schist  into  the  diorite, 
the  veins  invariably  narrow,  but  continue  well  defined.  In  the 
rhyolite-porphyry  the  same  fissures  lose  their  compact  character, 
and  split  into  net-works  of  fractures  through  which  the  ore  is 
dissipated;  a  well-defined  vein  upon  passing  into  quartzite  in  the 
Big  Seven  Mine  in  this  district  splits  into  many  small  stringers. 

The  Distinction  between  Intercalated  and  Fissure  Veins  in 
Schistose  Rocks. — This  distinction  is  of  great  importance  in  the 
investigation  of  quartz  veins  through  schists  or  gneisses,  which 
frequently  carry  intercalated  quartz  lenses,  or  beds  that  are 
deceptive  in  appearance  and  not  connected  with  the  important 
mineralization  of  the  district. 
1  W.  H.  Weed,  Trans.  A.  I.  M.  E.,  XXXI,  p.  637. 


68  EXAMINATION  OF  PROSPECTS 

In  a  district  where  quartz  lodes  later  than  the  schistosity  are 
known  to  exist,  and  a  quartz  body  conformable  to  the  schistosity 
is  found,  an  examination  of  the  lode  matter  in  thin  section  under 
the  microscope  will  usually  indicate  whether  the  deposit  is  later 
than  the  schistosity,  and  therefore  connected  with  the  important 
mineralization,  or  whether  it  is  older  than  the  mineralization  and 
forms  an  integral  part  of  the  metamorphic  series.  Under  the 
microscope  minutely  disseminated  minerals,  invisible  to  the  un- 
aided eye,  may  be  found  which  are  characteristic  of  the  commer- 
cially important  veins,  or,  on  the  other  hand,  such  minerals  or 
inclusions  may  be  lacking  and  the  quartz  under  the  microscope 
may  show  a  schistose  structure,  indicating  an  origin  contem- 
poraneous with  that  of  the  enclosing  beds. 

A  careful  investigation  in  the  field  will  often  disclose  branching 
stringers,  or  minute  veinlets,  which  indicate  a  later  origin  than 
that  of  the  schists.  Crustification  is  a  clear  indication  of  later 
origin. 

Fissures  and  Lodes  Formed  Subsequent  to  the  Principal  Miner- 
alization.— In  most  mining  districts  the  important  mineraliza- 
tion was  confined  to  one  period,  and  where  fissures  and  lodes 
of  various  origins  are  present,  it  is  of  the  greatest  importance  to 
distinguish  the  commercially  important  series  or  system  from  the 
others.  A  careful  plotting  of  all  the  fissures,  whether  ore-bearing 
or  barren,  will  often  indicate  the  prevailing  trend  of  the  pay 
fissures,  and  the  general  distribution  of  the  pay  fissures  about 
some  particular  intrusive,  or  in  some  definite  area  or  areas,  may 
afford  a  basis  upon  which  to  decide  whether  or  not  a  fissure  is 
worthy  of  exploration.  Lodes  of  different  dates  rarely  carry  the 
same  vein  fillings,  and  a  distinction  upon  this  basis  is  often 
practicable.  The  kind  and  degree  of  the  alteration  of  the  wall 
rocks  along  the  veins  affords  another  criterion. 


CHAPTER  IV 
PRIMARY  ORES  AND  THEIR  DISTRIBUTION 

Ore-deposits  of  commercial  grade  are  local  concentrations  of 
great  rarity  when  considered  in  relation  to  the  area  of  unmineral- 
ized  land  surfaces,  and  they  must  therefore  be  considered  as  the 
products  of  exceptional  and  complex  conditions.  The  data 
collected  from  a  great  number  of  individual  deposits  are  sufficient 
to  permit  certain  broad  generalizations  in  regard  to  the  conditions 
favorable  to  ore  deposition  which  even  numerous  exceptions  do 
not  invalidate. 

The  broadest  general  relation  of  ore-deposits  is  with  intrusive 
rocks,  and  while  there  are  notable  exceptions,  the  great  majority 
of  ore-deposits  have  a  visible  or  closely  inferred  connection  with 
intrusive  rock  masses. 

Conditions  that  permit  the  dissipation  of  the  ore-bearing 
vehicles  do  not  permit  the  concentration  of  metals  in  deposits; 
the  prime  factor  to  consider  in  this  connection  is  that  structural 
conditions  must  be  such  as  tend  to  concentrate  and  not  to  dissi- 
pate the  ore-bearing  solutions. 

The  disadvantage  of  extrusive  rocks  is  at  once  apparent;  any 
metals  the  magma  may  have  contained  are  dissipated  at  the 
surface  and  are  lost;  with  the  exception  of  tiny  vugs  and  cracks 
in  quickly  cooled  flows,  no  concentrations  of  metals  are  known 
in  extrusive  rocks,  and  the  lack  of  confinement  and  quick  cooling 
apparently  do  not  permit  extrusives  to  exert  mineralizing  effect 
upon  the  rocks  with  which  they  come  in  contact.  The  close 
mineralogical  similarity  between  extrusives  and  intrusives,  and 
the  great  difference  in  mineralizing  power  is  a  proof  of  the  ease 
with  which  magmas  part  with  their  metallic  content,  and  of  the 
mobility  of  the  ore-bearing  vehicles  in  comparison  with  the  parent 
magma. 

69 


70 


EXAMINATION  OF  PROSPECTS 


As  surface  flows  present  the  extreme  conditions  permitting 
dissipation,  so  laccoliths  probably  represent  the  extreme  con- 
ditions favorable  to  the  concentrated  action  of  ore-bearing 
vehicles.  The  intrusion  of  a  laccolith  causes  a  domal  uplift  of 
overlying  strata,  with  attendant  fissuring  of  these  strata;  the 
changes  of  volume  during  intrusion,  cooling  and  contraction  give 
rise  to  forces  that  reopen  or  keep  open  such  fissures  over  long 
periods  of  time,  and  afford  vents  for  the  mobile  constituents  of 
the  magma,  which  are  the  ore-bearing  vehicles;  in  an  extreme 
case  the  entire  emanation  from  a  laccolith  may  be  conceived  to 
pass  through  a  single  fissure. 


FIG.  40. — Ideal  section  of  laccoliths.     After  Gilbert. 

Between  these  two  extremes  of  dissipation  and  confinement  an 
infinite  variety  of  intrusions  are  found  in  nature  that  give  rise  to 
an  infinite  variety  of  conditions  under  which  ore  deposition  may 
take  place. 

In  general,  large  bodies  of. igneous  rocks  do  not  carry  ore 
deposits  through  their  masses,  the  tendency  being  for  the  deposits 
to  form  in  adjacent  rocks,  or  in  the  first  cooled  parts  or  edges  of 
the  intrusive  itself;  the  stresses  that  fissure  adjacent  rocks  are 
absorbed  in  the  still  hot  and  plastic  parts  of  the  intrusive,  but 
not  infrequently  shatter  the  already  solidified  peripheries  and 
afford  therein  channels  for  the  passage  of  the  ore-bearing  solu- 
tions and  favorable  loci  for  ore  deposition. 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  71 

Deep  intrusives  cut  through  and  come  in  contact  with  many 
older  rocks,  and  where  these  rocks  exert  precipitative  action  on  the 
ore-bearing  solutions  occur  other  favorable  loci  for  ore  deposition. 

In  general,  prerequisites  for  ore  deposition  are  conditions 
that  permit  the  escape  of  the  ore-bearing  vehicles  from  intrusive 
magmas  through  restricted  channels. 

Metallogenetic  Epochs  and  Provinces. — There  are  certain 
broad  divisions  in  North  America  of  which  particular  types  of 
deposits  are  characteristic.  These  divisions  in  general  may  be 
assigned  to  different  geologic  epochs.  Several  prominent  min- 
eral belts  are  well  known.  Gold  quartz  veins  in  schistose  rocks 
are  characteristic  of  a  zone  stretching  for  a  long  distance  through 
California  and  to  the  north  and  south.  A  series  of  lenticular 
copper  deposits  occurs  along  the  foot-hills  of  the  Sierra  Nevada 
in  California.  Many  silver  and  silver-gold  deposits  east  of  the 
Sierra  Nevadas  present  points  of  similarity  and  are  referred  to 
the  early  Tertiary;  these  deposits  are  characteristic  of  a  broad 
zone  that  persists  for  a  great  distance  to  the  south  through  the 
Cordilleras.  Certain  areas  along  the  Appalachians  in  the  Eastern 
States  and  in  eastern  Canada,  and  certain  areas  in  the  Western 
States,  are  characterized  by  deposits  of  pre-Cambrian  age,  which 
likewise  present  many  points  of  similarity.  The  distribution  of 
the  disseminated  chalcocite  enrichments  in  the  southwestern 
desert  region  is  well  known. 

A  majority  of  western  ore  deposits1  were  formed  in  late  Cre- 
taceous or  in  Tertiary  times,  and  no  great  epochs  of  dynamic 
stress  and  metamorphism  have  affected  them. 

The  Distribution  of  Ore -Deposits  in  Individual  Mining  Districts. 
—The  valuable  ore-deposits  of  any  mining  district  commonly  may 
be  referred  to  a  single  period  of  mineralization,  or  to  a  single  set  of 
conditions;  wherever  possible,  these  governing  factors  should  be 
determined  and  all  exploration  work  should  be  planned  with  re- 
gard to  them.  Exploration  by  elimination,  or  the  demonstration 
that  certain  areas  do  not  contain  ore-deposits,  rarely  yields 
results  in  proportion  to  the  money  spent;  all  exploration  should 

1  Waldemar  Lindgren,  Economic  Geology,  Vol.  IV,  p.  58. 


72  EXAMINATION  OF  PROSPECTS 

be  directed  to  test  the  hypothesis  that  appears  most  reasonable 
in  the  light  of  a  knowledge  of  ore-deposits  in  general,  and  of  the 
particular  mining  district  in  which  the  work  is  undertaken. 

The  principal  ore-deposits  of  any  mining  district  are  likely  to  be 
confined  to  a  certain  series  of  fissures,  to  some  particular  rock 
mass,  stratum,  area  of  altered  rocks,  locus  of  shattering,  or  to  the 
vicinity  of  some  particular  intrusive  or  contact;  not  infrequently, 
the  payable  veins  of  a  district  fall  into  groups  of  similar  strike, 
while  the  barren  veins  or  lean  veins  fall  into  other  groups.  The 
significance  of  these  relations  is  apparent  in  directing  exploration. 

The  Association  of  Ore -Deposits  with  Certain  Rocks. — There  is 
a  persistent  connection  between  ore-deposits  and  monzonitic 
rocks  throughout  the  Cordilleran  region;  the  examples  of  this 
relation  include  many  of  the  most  important  districts.  That 
the  converse  of  'this  relation — that  ore-deposits  may  be  expected 
where  monzonitic  stocks  are  found — is  not  true,  as  is  illustrated 
by  the  numerous  monzonite  masses  through  New  Mexico  that 
are  not  connected  with  any  important  mineralization. 

The  supposition  that  certain  types  of  igneous  rocks  indicate 
the  existence  of  ore  is  a  fallacy  that  has  been  the  cause  of  much 
fruitless  exploration  and  loss.  While  a  particular  intrusive 
frequently  controls  ore  deposition  over  a  limited  district  or  area, 
and  exploration  in  that  district  is  best  confined  to  the  sphere  of 
influence  of  the  intrusive,  it  does  not  follow  that  the  same  kind 
of  rock  elsewhere  is  a  favorable  indication  of  valuable  deposits. 

The  association  of  tin  and  of  tungsten  with  granite  is  well 
established,  as  is  also  the  association  between  nickel,  cobalt, 
platinum  and  chromium  with  basic  rocks,  commonly  those  that 
carry  abundant  ferro-magnesian  silicates.  Many  other  asso- 
ciations have  been  pointed  out,  but  they  appear  to  be  persistent 
in  restricted  areas  only. 

The  Depth  to  which  Primary  Ores  Persist. — The  outcrop  or  the 
exposure  at  any  horizon  of  a  primary  ore-deposit  affords  a  cri- 
terion of  the  value  of  that  deposit  to  any  depth  above  the  zone  of 
primary  impoverishment;  unless  it  can  be  shown  that  the  deposit 
represents  the  root-of  a  vein  by  far  the  greater  proportion  of  which 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  73 

v 

has  been  destroyed  by  erosion,  this  is  likely  to  mean  any  depth 
attainable  by  mining  operations. 

There  is  no  genetic  reason  why  the  values  of  any  'primary 
deposit  should  not  continue  to  great  depths.  Veins  that  are 
pockety,  or  whose  ore-shoots  are  short  and  irregular,  will  prob- 
ably maintain  these  characteristics  in  depth,  but  the  values, 
whatever  their  distribution,  should  be  substantially  the  same 
at  all  horizons. 

Secondary  ores,  however,  being  the  results  of  surface  processes, 
are  limited  to  horizons  near  the  surface;  in  mining  geology  there 
is  no  distinction  of  greater  practical  importance  than  that 
between  primary  and  secondary  ores. 

The  Criteria  of  Primary  Ores. — In  the  investigation  of  any 
ore-shoot  the  first  consideration  is  whether  the  ore  is  primary, 
secondary,  or  residual,  as  upon  this  rests  all  conclusions  in  regard 
to  its  persistency  in  depth.  A  primary  ore  is  an  ore  that  has 
undergone  no  change  since  deposition.  A  secondary  ore  is  an 
ore  formed  by  secondary,  or  surface,  agencies.  A  residual  ore  is 
an  ore  that  has  remained  after  the  solution  and  removal  of  asso- 
ciated minerals  by  secondary  processes. 

In  distinguishing  between  primary  and  secondary  ores,  the 
first  criterion  is  the  presence  or  absence  of  signs  of  oxidation;  if 
an  ore  carries  traces  of  oxidation,  secondary  action  must  be  sus- 
pected, although  it  may  be  shown  that  it  has  had  no  effect  in  the 
distribution  of  values. 

In  thin  section  under  the  microscope  the  manner  of  intergrowth 
of  primary  minerals  is  characteristic,  and  in  this  way  primary  and 
secondary  ores  may  usually  be  distinguished  from  each  other; 
evidence  of  structural  intergrowth  is  rarely  visible  to  the  unaided 
eye.  The  presence  of  two  generations  of  sulphides,  the  richer 
being  in  general  the  later,  and  coating  the  other  as  if  precipitated 
upon  it,  is  usually  clear  evidence  of  secondary  enrichment.  The 
presence  of  seamlets  of  one  sulphide,  especially  if  the  richer, 
through  another  sulphide,  is  likewise  evidence  of  secondary 
enrichment.  In  deposits  of  massive  pyrite  that  carry  chalco- 
pyrite  of  undoubted  primary  origin,  there  is  a  tendency  for  the 


74  EXAMINATION  OF  PROSPECTS 

latter  mineral  to  occur  as  veinlets  through  the  pyrite.  This  is 
probably  due  to  the  greater  solubility  and  the  later  crystalliza- 
tion of  chalcopyrite  over  pyrite  under  conditions  of  regional 
metamorphism,  to  which  most  of  the  deposits  of  this  kind  have 
been  subjected. 

The  presence  of  fluid  inclusions  in  an  ore  is  evidence  of  primary 
origin  unless  secondary  processes  are  seen  to  have  been  at  work; 
a  banded  structure,  or  crustification,  is  another  evidence  of 
primary  origin.  While  there  are  many  doubtful  cases  where  an 
ore  may  not  be  assigned  definitely  to  either  primary  or  secondary 
processes,  in  a  majority  of  ores  the  typical  associations  of  primary 
minerals,  their  manner  of  intergrowth,  the  presence  of  fluid 
inclusions  or  of  crustification,  indicate  a  primary  ore,  while  the 
absence  of  oxidation,  of  secondary  minerals,  or  of  secondary 
rearrangement  of  the  primary  minerals,  indicate  that  surface 
agencies  have  played  no  part  in  the  distribution  of  values; 
such  an  ore  may  be  expected  to  continue  in  depth  to  the  zone  of 
primary  impoverishment,  which  in  most  cases  is  deeper  than  the 
limits  of  profitable  mining. 

No  absolute  rule  may  be  formulated  for  field  use,  but  an  asso- 
ciation of  the  following  sulphides  commonly  indicates  a  primary 
origin  for  any  ore  that  carries  no  trace  of  oxidation  or  typical 
secondary  structure:  galena,  zincblende  or  chalcopyrite  with 
pyrite,  pyrrhotite  or  arsenopyrite.  The  most  common  primary 
occurrence  of  gold  is  either  as  native  gold  or  as  a  telluride. 
Probably  the  most  common  primary  occurrence  of  silver  is  as 
argentite.  Auriferous  or  argentiferous  tetrahedrite  is  a  common 
primary  mineral. 

The  Minerals  of  Distinctively  Primary  Origin. — The  minerals 
present  in  an  ore  frequently  afford  a  basis  upon  which  to  judge 
its  origin.  Some  minerals  are  distinctively  primary,  some  dis- 
tinctively secondary,  others,  and  among  them  are  some  of  the 
most  important  ore  minerals,  are  in  some  instances  primary  and 
in  others  secondary.  The  presence  of  a  mineral  of  secondary 
origin  proves  the  action  of  surface  agencies;  a  mineral  that  is 
known  to  be  sometimes  of  secondary  origin  indicates  that  surface 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  75 

agencies  may  have  enriched  the  ore  under  consideration,  and  so 
casts  doubt  upon  the  primary  character  of  the  ore  containing  it. 
Genetic  classifications  of  minerals  have  been  made  by  Waldemar 
Lindgren1  and  W.  H.  Emmons,2  and  their  tables,  which  were 
made  for  another  purpose,  have  been  freely  consulted  in  the 
preparation  of  the  following  lists: 

Distinctively  Primary  Minerals 

ORE  MINERALS 

Arsenopyrite  Pyrrhotite 

Bismuthinite  Tellurides 

Cobaltite  Tetradymite 
Stibnite 

GANGUE  MINERALS 

Albite  Orthoclase 

Biotite  Rhodonite 

Diopside  Rutile 

Fluorite  Scapolite 

Garnet  Specularite 

Graphite  Spinel 

Hornblende  Topaz 

Ilmenite  Tourmaline 
Muscovite  ' 

Minerals  both  Primary  and  Secondary  in  Origin 

Argentite  Pfoustite 

Bornite  (usually  secondary)  Pyrite 
Chalcopyrite  (usually  primary)   Polybafeite 

Enargite  Sphalerite 

Galena  Stephanite 

Gold  Tetrahedrite 

Pyrargyrite  Tennantite 

1  Economic  Geology,  Vol.  II,  p.  122. 

2  Economic  Geology,  Vol.  Ill,  p.  625. 


76  EXAMINATION  OF  PROSPECTS 

Distinctively  Secondary  Minerals 

Chalcedony  Pyrolusite 

Cuprite  Chlorides 

Chalcocite  Sulphates  of  the  heavy  metals. 

Covellite  Carbonates  of  the  heavy  metals. 

Kaolin  Phosphates  of  the  heavy  metals. 

Limonite  Silicates  of  the  heavy  metals. 

Opal  Arsenates  of  the  heavy  metals. 

The  Primary  Associations  of  Metals. — In  primary  ores  some 
metals  exhibit  a  tendency  to  associate  themselves  with  certain 
minerals.  Among  the  more  prominent  of  these  primary  asso- 
ciations are: 

Gold  with  quartz. 
Gold  with  pyrite. 
Gold  with  chalcopyrite. 
Silver  with  galena. 
Silver  with  copper. 
Silver  with  manganese. 
Copper  with  pyrite. 
Lead  with  barium. 

Of  these  associations  that  of  gold  with  chalcopyrite  is  probably 
stronger  than  its  association  with  either  quartz  or  pyrite,  and  the 
'association  of  silver  with  lead  is  most  marked. 
*  The  Accessory  Minerals  that  Commonly  Indicate  a  Segregation 
of  Values. — Tetrahedrite  is  a  guide  to  high  silver  and  gold  values 
in  most  deposits  in  which  it  occurs.  In  quartz  veins,  the  presence 
of  finely  disseminated  galena  or  chalcopyrite  ("sulphurets ") ,  or 
the  presence  of  fluorite,  are  often  indicative  of  high  gold  or  silver 
values.  In  quartz  veins  that  carry  gold  and  silver  it  is  frequent 
that  quartz  of  a  certain  texture  carries  high  values,  while  asso- 
ciated quartz  of  other  textures  is  low  grade  or  barren. 

It  is  generally  supposed  that  galena  having  curved  crystal 
faces  carries  higher  silver  values  than  galena  of  cubical  cleavage; 
it  is  not  unusual  that  where  the  crystal  faces  of  galena  are  curved 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  77 

the  associated  minerals  have  a  similar  structure;  the  supposed 
relation,  therefore,  is  not  always  a  reliable  guide.  The  fact  is 
well  known  that  a  fine  grained  or  granular  galena  is  likely  to  carry 
more  silver  than  the  coarsely  or.  well-crystallized  mineral.  Well- 
crystallized  pyrite  is  usually  quite  lean  in  copper.  It  seems 
probable  that  the  presence  of  silver  in  galena  and  of  copper  in 
pyrite  tend  to  interrupt  crystallization,  and  it  is  not  unusual  that 
information  as  to  the  content  of  these  associated  or  contained 
metals  may  be  gained  by  an  inspection  of  the  casts  left  in  the 
outcrop  after  the  solution  and  removal  of  the  principal  sulphide. 

The  relations  between  segregations  of  values  and  accessory 
minerals,  or  varying  textures,  are  soon  learned  in  the  study  of 
individual  deposits,  and  often  form  valuable  guides  in  exploration. 

Primary  Gold  Ores. — AT  CRIPPLE  CREEK,  COLORADO.1 — The 
characteristic  feature  of  the  ores  is  the  occurrence  of  the  gold  in 
combination  with  tellurium,  chiefly  as  calaverite,  but  partly  also 
as  the  more  argentiferous  sylvanite,  and  probably  to  a  minor  ex- 
tent as  other  gold,  silver  and  lead  tellurides.  Native  gold  appears 
to  be  absent  from  the  telluride  ores,  except  where  set  free  by  oxi- 
dation. Pyrite  is  widely  disseminated  through  the  country  rock 
and  also  occurs  in  small  quantities  in  the  fissures  associated  with 
tellurides.  Galena  and  sphalerite  are  sparingly  present  in  the 
majority  of  the  veins;  tetrahedrite  and  stibnite  are  of  frequent 
occurrence;  molybdenite  in  small  quantities  is  probably  always  . 
present.  The  tetrahedrite  is  usually  rich  in  silver,  and  also  con-  * 
tains  gold;  possibly,  however,  the  latter  is  due  to  admixed  calav- 
erite, as  the  two  minerals  are'often  found  in  intimate  intergrowth. 
The  galena  and  zincblende  rarely  contain  enough  of  the  precious 
metals  to  form  ore.  Auriferous  pyrite  is  often  reported,  but  in 
the  cases  investigated  the  gold  was  found  to  be  present  as  admixed 
tellurides.  The  usual  minerals  of  the  scanty  gangue  are  quartz, 
fluorite,  and  dolomite. 

AT  THE  ALASKA-TREADWELL  MINE,  ALASKA.2 — The  ore-bodies 
follow  a  dike  of  albite-diorite  that  carries  a  net.-w.prk  of  quartz  and 

1  Lindgren  and  Ransome,  U.  S.  G.  S.,  P.  P.  54,  p.  169. 

2  A.  C.  Spencer,  U.  S.  G.  S.  Bull.  225,  p.  39,  and  Bull.  287,  p.  105. 


78  EXAMINATION  OF  PROSPECTS 

calcite  veinlets.  Pyrite,  occurs  both  in  the  veinlets  and  dis- 
seminated through  the  rock  itself.  Gold  occurs  in  association 
with  the  pyrite  and  also  native,  and  a  large,  though  variable, 
proportion  of  the  value  of  the  ore  is  saved  by  amalgamation. 
Visible  specks  of  the  gold  are  sometimes,  though  rarely,  found. 
Associated  minerals  always  present  are  pyrrhotite  and  magnetite ; 
molybdenite  is  of  common  occurrence.  Native  arsenic,  realgar, 
and  orpiment  have  been  noted.  Arsenopyrite  is  suspected. 
Stibnite  occurs  in  small  amounts  with  the  quartz.  The  bullion 
assays  indicate  small  quantities  of  silver  only. 

AT  GRASS  VALLEY,  CALIFORNIA.1 — The  primary  ore  is  quartz 
that  carries  free  gold  in  both  fine  and  coarse  particles,  with  from 
2  per  cent,  to  3  per  cent,  of  sulphides  which  also  carry  gold.  Py- 
rite is  the  predominate  sulphide;  associated  with  it  are  galena, 
zinc-blende,  chalcopyrite,  and  arsenopyrite.  Subordinate  acces- 
sory minerals  are  tetrahedrite  and  molybdenite.  The  quartz 
carries  a  little  calcite.  Fluid  inclusions  are  abundant,  and  in 
many  specimens  are  distributed  in  a  manner  dependent  upon  the 
distribution  of  the  sulphides  through  the  quartz. 

Primary  Copper  Ores. — AT  CLIFTON,  ARIZONA,  the  primary 
ores  are  unpayable  disseminations  that  assay  about  3/10  of  1  per 
cent,  copper;  from  these  low-grade  ores  the  valuable  deposits 
have  been  formed  by  secondary  enrichment.  The  primary  ore 
consists  of  sericitized  quartz-monzonite  porphyry  that  carries 
veinlets  of  quartz  and  pyrite,  disseminated  pyrite,  and  a  little 
finely  divided  chalcopyrite,  zincblende,  and  molybdenite. 

AT  DUCKTOWN,  TENNESSEE,2  the  primary  ores  consist  of  mass- 
ive pyrrhotite  containing  particles  and  stringers  of  chalcopyrite 
and  pyrite,  together  with  minute  quantities  of  galena  and  zinc- 
blende.  Calcite,  zoisite  and  quartz,  and  occasionally  bunches  of 
garnet  occur  with  the  ore. 

IN  SHASTA  COUNTY,  CALIFORNIA/  the  primary  ores  consist  of 

1  Waldemar  Lindgren,  Seventeenth  Annual  Report  U.  S.  G.  S.,  Pt.  II,  p. 
124. 

2  W.  H.  Weed,  "Copper  Mines  of  the  World,"  p.  349. 

3  J.  S.  Diller,  Bull.  285,  U.  S.  G.  S.,  p.  173. 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  79 

pyrite  and  chalcopyrite,  with  some  zincblende  and  galena, 
associated  with  quartz,  calcite,  and  barite. 

AT  FALUN,  SWEDEN/  the  ore  is  essentially  a  granular-crystal- 
line mixture  of  pyrite  and  quartz,  with  accessory  magnetite, 
chalcopyrite,  pyrrhotite,  zincblende,  and  in  rare  cases,  galena. 

AT  BISBEE,  ARIZONA,2  the  primary  ore  consists  of  pyrite  con- 
taining variable  amounts  of  chalcopyrite  and  a  little  sphalerite 
associated  with  calcite,  amphibole,  pyroxene,  garnet,  chlorite, 
quartz,  and  vesuvianite;  the  metamorphic  silicates  are  usually  so 
finely  divided  as  to  be  indistinguishable  by  the  unaided  eye.  The 
primary  ore  in  general  is  unpayable. 

Contact  Ores. — Typical  contact  ores  carry  pyrite,  chalco- 
pyrite, bornite,  pyrrhotite,  specularite,  and  magnetite,  with  com- 
monly lesser  amounts  of  galena  and  zincblende,  associated  with 
garnet,  wollastonite,  epidote,  amphibole,  pyroxene,  vesuvianite, 
quartz  and  calcite.  The  gold  and  silver  content  is  usually  low. 
The  distinctive  association  of  minerals  in  contact  ores  is  that  of 
primary  oxides  with  sulphides.3 

AT  SAN  PEDRO,  NEW  MEXICO/  the  contact  ores  consist  of 
massive  garnet  replacing  limestone  and  carrying  chalcopyrite, 
specularite,  epidote,  vesuvianite,  wollastonite,  quartz  and  calcite. 

Primary  Silver  Ores. — AT  LAKE  CITY/  COLORADO,  the  primary 
ore  minerals  are  galena,  tetrahedrite,  chalcopyrite,  sphalerite  and 
pyrite,  associated  with  quartz,  rhodonite,  rhodochrosite,  and 
barite.  The  silver  is  contained  in  the  galena  to  the  extent  of 
22  to  30  oz.  per  ton,  and  in  the  tetrahedrite,  which  is  probably 
related  to  freibergite,  in  much  larger  quantity. 

AT  THE  GRANITE-BIMETALLIC  MINE,  MONTANA/  the  primary 
ore  consists  of  pyrite,  arsenopyrite,  tetrahedrite,  and  tennantite, 
with  lesser  quantities  of  galena  and  zincblende,  in  a  gangue  of 

1  "Nature  of  Ore  Deposits,"  Beck- Weed,  p.  460. 

2  F.  L.  Ransome,  P.  P.  21,  U.  S.  G.  S. 

3  Waldemar  Lindgren. 

4  M.  B.  Yung  and  R.  S.  McCaffery,  Trans.  A.  I.  M.  E.,  XXXIII,  p.  355. 

5  J.  D.  Irving,  Bull.  260,  U.  S.  G.  S.,  p.  81. 

8  W.  H.  Emmons,  Bull.  315,  U.  S.  G.  S.,  p.  39. 


80  EXAMINATION  OF  PROSPECTS 

quartz  and  rhodochrosite.  Sparingly  scattered  through  this 
ore  are  found  small  specks  of  pyrargyrite,  proustite,  and,  rarely, 
realgar  and  orpiment.  This  ore  carries  from  20  to  30  oz.  silver 
and  from  $1.50  to  $3.00  in  gold. 

AT  GEORGETOWN,  COLORADO, *  the  primary  ore  consists  of 
argentiferous  galena  and  zincblende,  with  cupriferous  pyrite,  and 
chalcopyrite,  in  a  gangue  of  quartz,  and  siderite,  rhodochrosite, 
dolomite  and  calcite,  in  varying  proportions;  fluorite  is  present, 
but  is  rare. 

AT  TONOPAH,  NEVADA/  the  primary  ores  consist  of  argentite, 
polybasite,  stephanite,  and  gold  in  a  still  undetermined  form, 
associated  with  chalcopyrite,  pyrite,  and  subordinate  galena  and 
zincblende  in  a  gangue  of  quartz,  adularia,  sericite  and  carbonates. 

AT  PARK  CITY,  UTAH/  the  primary  ores  consist  of  galena 
tetrahedrite,  pyrite,  chalcopyrite  and  zincblende  in  a  siliceous 
gangue  that  carries  a  little  barite  and  fluorite. 

AT  PACHUCA,  MEXICO/  the  primary  ores  where  the  veins 
become  impoverished  in  depth  consist  of  galena,  zincblende,  and 
pyrite  in  a  gangue  of  quartz  and  rhodonite. 

AT  GUANAJUATO,  MEXICO, 5  the  primary  ores  consist  of  tetrahe- 
drite with  zincblende  and  pyrite  in  a  gangue  of  quartz,  calcite, 
rhodonite,  and  fluorite. 

Primary  Lead  Ores. — AT  LEADVILLE,  COLORADO, 6  the  primary 
ores  consist  of  limestone  replaced  by  quartz,  pyrite,  galena  and 
zincblende,  carrying  a  small  quantity  of  silver  sulphide.  The  ore 
contains  about  1  per  cent,  manganese,  in  what  form  is  not  stated, 
as  rhodonite  and  rhodochrosite  are  absent. 

Lv  THE  COEUR  D'ALENE  DISTRICT,  IDAHO/  the  primary  ore 
consists  of  galena,  pyrite,  pyrrhotite,  chalcopyrite,  sphalerite, 

1  J.  E.  Spurr,  P.  P.  63,  U.  S.  G.  S.,  p.  136. 

2  J.  E.  Spurr,  P.  P.  42,  U.  S.  G.  S.,  p.  22. 

3  J.  M.  Boutwell,  Bull.  225,  U.  S.  G.  S.,  p.  147. 

4  Srs.  Aguilera  and  Ordonez,  Trans.  A.  I.  M.  E.,  XXXII,  p.  224. 

5  Beck- Weed,  "Nature  of  Ore  Deposits,"  p.  265. 

6  S.  F.  Emmons,  Mono.  XII,  U.  S.  G.  S.,  p.  32. 

7  F.  L.  Ransome,  P.  P.  62,  U.  S.  G.  S.,  p.  107, 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  81 

and  a  little  tetrahedrite  and  stibnite.  Siderite  and  a  little 
quartz  form  the  gangue.  Tetrahedrite  is  commonly  associated 
with  high  values  in  silver. 

Primary  Zinc  Ores. — The  only  primary  ore  of  zinc  of  impor- 
tance is  zincblende;  in  characteristic  occurrences  it  is  associated 
with  pyrite,  and  occasionally  with  galena  and  chalcopyrite. 

The  Depth  of  Primary  Ore  Deposition. — An  important  factor  in 
the  consideration  of  the  probable  behavior  of  a  primary  deposit 
in  advance  of  exploration  is  the  depth  at  which  it  formed.  If  it 
can  be  shown  that  a  deposit  was  formed  at  the  present  surface, 
then  it  is  evident  that  the  type  of  ore  prominent  at  the  surface 
cannot  be  expected  to  continue  in  depth,  as  the  conditions  under 
which  the  ore  was  deposited — namely,  surface  conditions — were 
absent. 

A  deposit  formed  at  relatively  shallow  depth  is  likely  to  grow 
more  regular  with  deeper  exploration;  stringers  and  branch  veins 
commonly  consolidate  in  a  single  lode,  or  lodes,  of  relatively 
greater  uniformity  of  dip,  strike  and  thickness  as  compared  with 
the  more  scattered  units  near  the  surface.  This  advantage  of 
regularity  is  likely  to  be  offset  by  a  decrease  in  size  in  the 
deeper  parts  of  the  deposit. 

A  deposit  of  deep-seated  origin,  however,  owes  its  discovery  to 
a  deep  erosion,  which  has  presumably  removed  the  upper  and 
irregular  portions  of  the  deposit,  and  exposed  the  deeper  zone  of 
relatively  greater  regularity.  By  regularity  is  meant  the  rela- 
tive regularity  of  different  parts  of  the  same  deposit,  and  not  the 
absolute  regularity  of  a  particular  deposit  as  compared  with 
deposits  in  general.  Some  primary  deposits  are  probably  quite 
irregular  at  all  depths.  A  deposit  of  deep-seated  origin  whose 
values  have  not  been  redistributed  by  surface  agencies  is  likely 
to  be  persistent  in  depth  down  to  the  zone  .of  primary  impover- 
ishment, and  any  section  of  such  a  deposit  may  be  considered  as 
indicative  of  its  character  at  other  horizons,  in  the  absence  of  the 
factors  that  cause  the  localization  of  values  into  ore-shoots. 

The  deepest  parts  of  ore-deposits,  usually  referred  to  as  the 
roots  of  the  veins,  are  not  so  readily  recognizable  as  the  zones 

6 


82  EXAMINATION  OF  PROSPECTS 

that  have  been  referred  to,  and  the  data  in  regard  to  them  are 
less  satisfactory. 

That  increasing  depths  should  give  rise  to  transition  types  is 
to  be  expected,  and  it  is  not  possible,  therefore,  to  divide  de- 
posits into  sharply  delimited  classes  on  the  basis  of  their  depths 
of  formation. 

Deposits  Formed  at  the  Surface.1 — Primary  mineral  deposits 
formed  at  the  surface  by  hot  waters  are  rarely  of  economic 
importance;  their  primary  condition  is  commonly  obscured  by 
the  action  of  surface  agencies.  The  sinters  characteristic  of 
surface-formed  deposits  are  commonly  made  up  of  silica,  as  opal 
or  chalcedony,  and  earthy  carbonates.  Calcite,  fluorite,  celes- 
tite,  barite,  and  many  other  gangue  minerals  may  also  develop 
in  crystallized  form.  Stibnite,  pyrite,  marcasite  and  cinnabar 
are  known  in  crystallized  form,  and  many  other  sulphides  have 
been  detected  chemically  in  such  deposits.  Surface  waters 
containing  atmospheric  oxygen  are  likely  to  have  altered  these 
deposits  greatly,  and  limonite,  hydrous  oxides,  carbonates,  and 
sulphates  of  the  heavy  metals  predominate  among  the  ore  min- 
erals, and  kaolin,  allophane  and  chloropal  among  the  gangue 
minerals.  The  surficial  formation  of  these  deposits  is  commonly 
indicated  by  their  structure. 

Veins  Formed  near  the  Surface. — In  typical  examples  these 
veins  cut  through  beds  of  relatively  recent  volcanic  rocks,  and 
their  depths  at  the  time  of  vein  formation  may  usually  be  deter- 
mined with  fair  accuracy.  Structural  features  indicating  a 
formation  at  shallow  depth  are:2  a  greater  number  and  width 
of  the  fissures  near  the  surface;  a  branching  of  the  upper  parts  of 
fissures;  and  fissures  of  changing  dip,  of  which  the  deeper  part 
is  likely  to  have  the  flatter  dip. 

Metasomatic  alteration  of  wall  rocks  is  likely  to  extend  to 
greater  distances  from  veins  formed  near  the  surface  than  from 
deeper  veins.  In  rocks  of  medium  acidity  a  strong  sericitization 

1  This  and  succeeding  paragraphs  are  taken  from  Mr.  Waldemar  Lindgren's 
articles  in  Economic  Geology,  Vol.  II,  p.  460,  and  Economic  Geology,  Vol.  I, 
p.  34. 

2  Waldermar  Lindgren,  P.  P.  54,  U..S.  G.  S.,  p.  167. 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  83 

is  common  immediately  along  the  veins,  and  extensive  pyritiza- 
tion  is  also  frequent;  the  altering  solutions  have  a  tendency  to 
spread  from  the  veins  through  a  large  area  of  adjoining  rocks, 
owing  to  more  extensive  fissuring  near  the  surface,  where,  robbed 
of  their  most  active  ingredients,  they  effect  a  propylitic  alteration 
over  large  areas.  In  very  basic  rocks  this  propylitization  extends 
close  up  to  the  veins,  where  sericitization  is  likely  to  take  its  place ; 
in  large,  irregularly  altered  areas,  especially  in  siliceous  rocks  like 
rhyolite,  extensive  silicification  is  common. 

In  these  deposits  gold  and  silver  prevail,  and  as  compared  with 
deep-seated  veins  of  quartz  gangue,  silver  is  relatively  more 
abundatit,  and  free  gold  is  commonly  present  in  a  more  finely 
divided  form;  pyrite,  zincblende,  chalcopyrite,  arsenopyrite, 
argentite,  tellurides  and  stibnite  are  the  prevailing  ore  minerals; 
among  gangue  minerals,  quartz  is  most  abundant,  but  it  is  often 
accompanied  by  chalcedony  or  opal;  calcite  and  dolomite  are 
moderately  abundant  in  the  vein  filling;  siderite  occurs  more 
rarely;  barite  and  fluorite  predominate  locally.  Magnetite  and 
specularite,  as  well  as  all  the  silicates  belonging  to  the  greater 
depths  of  ore  deposition,  are  absent.  In  these  veins  the  filling 
of  open  spaces  is  an  important  process. 

Veins  of  Deep-seated  Origin. — These  veins  are  divided  by  Mr. 
Lindgren  into  four  classes: 

(a)  Contact  deposits,  which  are  discussed  elsewhere. 

(b)  Cassiterite  veins.  The  characteristic  minerals  of  this  type 
are  cassiterite,   pyrite,  arsenopyrite,  specularite,   quartz,  tour- 
maline, topaz,  lepidolite,  muscovite,  apatite,  fluorite,  and  wol- 
framite, with  subordinate  calcite  and  siderite.     These  veins  are 
commonly  poor  in  gold  and  silver,  and  the  metasomatic  altera- 
tion along  their  walls  is  likely  to  be  intense. 

(c)  Apatite  veins.  The  characteristic  minerals  of  this  type  are 
apatite  and  other  phosphates,   scapolite,   diopside,  hornblende, 
biotite,  specularite  and  pyrrhotite;  strong  metasomatic  action  is 
usual  along  the  walls  of  these  veins,  and  the  introduction  of 
chlorine  and  fluorine  is  characteristic;  these  veins  are  commonly 
poor  in  gold  and  silver. 


84 


EXAMINATION  OF  PROSPECTS 


(d)  Deep-seated  gold  and  silver  veins.  The  characteristic 
minerals  of  this  type  are  gold,  pyrite,  pyrrhotite,  galena,  zinc- 
blende,  magnetite,  specularite,  ilmenite,  quartz,  biotite,  tourma- 
line, garnet,  hornblende,  chlorite,  apatite,  spinell,  and  epidote; 
calcite  is  present  in  small  amounts;  the  replacement  of  the  coun- 
try rock  is  usually  strongly  marked;  amphibolites  and  micaceous 
schists  are  replaced  by  tourmaline,  garnet,  a  green  biotite  and 
epidote;  soda-lime  feldspars  are  unstable  under  the  influence  of 
vein  forming  solutions  and  alkali  feldspars  usually  do  not  form. 
These  veins  commonly  occur  in,  or  close  to,  granite  intrusives  in 
schists. 


FIG.  41. — Section  of  the   Valley   View  vein,   Tonopah,   Nevada,   showing 
upward  branching.     After  Spurr. 

As  a  rule,  the  walls  of  deep-seated  veins  are  not  altered  through 
so  great  distances  as  is  common  with  vein's  formed  at  shallow 
depths,  and  their  vein  fillings  frequently  bear  evidence  of  having 
been  subjected  to  the  stresses  of  dynamometamorphism.  The 
time  of  formation  of  the  deep-seated  deposits  is  likely  to  be 
remote  as  compared  with  deposits  formed  at  shallow  depths. 


PRIMARY  ORES  AND  THEIR  DISTRIBUTION  85 

Relative  Susceptibility  of  Hanging-  and  Foot -walls  to  Mineraliza- 
tion.— In  many  veins  the  hanging-wall  has  been  subject  to  brec- 
ciation  and  mineralization  to  a  far  greater  extent  than  the  foot- 
wall,  and  hydrothermal  alteration  more  frequently  extends  into 
it  than  into  the  foot-wall.  This  is  probably  the  result  of  the 
superior  resistance  of  the  foot-wall  to  fracturing  as  compared  with 
the  hanging-wall.  The  hanging- w^all  rocks  readily  adjust  them- 
selves to  stress  through  fracturing,  while  the  foot-wall,  under 
as  great,  or  greater,  stress,  remains  massive,  owing  to  the  rein- 
forcement by  underlying  rock  masses. 

© 


CHAPTER  V 
TYPES  OF  PRIMARY  ORE -DEPOSITS 

The  classification  here  used  is  one  of  convenience  only;  it  is 
not  intended  to  include  all  known  types  of  ore-deposits.  The 
characteristic  features  of  the  several  well-marked  types  of  pri- 
mary mineralizations  are  described  without  reference  to  the 
ultimate  source  of  their  metals.  The  minerals  of  many  deposits 
of  obscure  origin  were  probably  introduced  through  fissures  that 
are  now  healed,  or  are  due  to  the  migration  of  metals  from  such 
deposits.  A  discussion  of  the  genesis  of  these  deposits  is  not 
justified  by  the  present  state  of  knowledge  of  economic  geology. 

Magmatic  Segregations.— During  the  solidification  of  magmas 
under  conditions  that  do  not  permit  the  escape  of  their  metallic 


200 300  400  500  feet 


FIG.  42. — Sketch  showing  the  Drinkwater  zone  of  quartz  lenses  in  alaskite, 
Silver  Peak,  Nevada;  the  lenses  are  magmatic  segregations.     After  Spurr. 

content,  there  is  a  tendency  for  like  particles  to  form  segregations 
probably  through  mass  action,  or  the  mutual  attraction  of  like 
particles.  Magmatic  segregations  are  commonly  made  up  of 
compounds  of  relatively  low  mobility. 

Basic  rocks  appear  to  be  the  most  suitable  for  the  formation  of 
segregations,  the  characteristic  minerals  of  which  are  magnetite, 
ilmenite,  chromite,  pyrrhotite,  pyrite,  and  pentlandite.  The 
gangue  minerals  are -those  of  the  containing  rock.1  Acid,  or 

1  Waldemar  Lindgren,  Economic  Geology,  Vol.  II,  p.  110. 

86 


TYPES  OP  PRIMARY  ORE-DEPOSITS  87 

quartzose  segregations,  however,  are  known.  Gold  and  plati- 
num have  been  established  as  constituent  minerals  of  rock  masses, 
chiefly  as  sparse  disseminations,  unpayable  in  themselves,  but 
important  as  the  sources  of  placer  deposits. 

Magmatic  segregations  are  rarely  of  economic  importance  as 
ores,  and  may  usually  be  recognized  in  thin  section  under  the 
microscope  by  the  manner  of  intergrowth'  of  the  ore  with  the 
rock  minerals.  It  is  not  unusual  to  find  that  rocks  containing 
such  deposits  have  become  further  differentiated  into  two  or  more 
parts  of  different  mineralogical  composition.  Clearly  established 
examples  of  magmatic  segregations  are  more  rare  than  was  at 
one  time  supposed.  The  most  generally  accepted  type  is  that  of 
the  titaniferous  magnetite  deposits. 

IN  THE  ADIRONDACK  MOUNTAINS,  NEW  'YORK,1  deposits  of 
titaniferous  magnetite  are  associated  with  basic  intrusives  in  such 
a  way  as  to  indicate  that  the  ore  minerals  were  segregated  from 
the  containing  rock  as  it  cooled.  No  ore  was  formed  along  the 
contacts  of  these  intrusives  with  the  enclosing  rocks,  and  the  ore 
grades  into  the  containing  rock  through  a  transition  zone  that 
shows  a  gradual  change  in  the  quantities  but  not  in  the  kinds  of 
the  constituent  minerals. 

AT  SILVER  PEAK,  NEVADA/  auriferous  quartz  appears  to  have 
segregated  from  enclosing  alaskite,  which  is  a  phase  of  the  accom- 
panying granite;  the  ore  bears  the  same  relation  to  the  alaskite 
that  the  latter  bears  to  the  granite.  The  alaskite  is  a  granite 
without  biotite;  the  quartz  is  an  alaskite  without  feldspar.  The 
segregated  quartz  is  in  this  case  of  the  same  age  and  generation 
as  the  granules  of  quartz  that  make  up  a  large  proportion  of  the 
alaskite  and  granite. 

Contact  Deposits. — Contact  deposits  are  deposits  formed  along 
the  contacts  between  intrusives  and  their  enclosing  rocks,  or  in 
these  rocks  in  immediate  proximity  to  the  intrusives.  They  are 
the  result  of  direct  emanation  of  mineral-bearing  solutions  from 

1  J.  F.  Kemp,  Nineteenth  Annual  Report,  U.  S.  G.  S.,  Ill,  p.  392. 
3  J.  E.  Spurr,  P.  P.  55.  p.  87. 


88  EXAMINATION  OF  PROSPECTS 

the  intruding  magma.  Contact  deposits  are  usually  limited  to 
rocks  that  exert  strong  precipitative  action.  The  characteristic 
minerals  of  contact  deposits  are:1  specularite,  magnetite,  bornite, 
chalcopyrite,  pyrite,  pyrrhotite,  and  more  rarely,  galena  and 
zincblende,  associated  with  garnet,  wollastonite,  epidote,  ilvaite, 
amphibole,  pyroxene,  zoisite,  vesuvianite,  quartz  and  calcite, 
and  rarely,  fluorite  and  barite.  The  unique  feature  is  the  associa- 
tion of  the  oxides  of  iron  with  sulphides.  The  sulphides  fre- 
quently carry  gold  and  silver,  but  usually  in  small  quantities 
only. 

Contact  deposits  are  rarely  of  commercial  importance,  although 
there  are  certain  notable  exceptions.  Many  deposits  formerly 
included  in  this  class  are  now  more  correctly  considered  as 
replacement  deposits. 

Contact  deposits  are  usually  quite  irregular  in  form,  due  largely 
to  the  common  irregularity  of  the  igneous  intrusion,  and  when 
the  ore  is  lost,  it  is  generally  recovered  with  difficulty;  while 
there  is  apparently  no  genetic  reason  why  contact  deposits, 
which  are  of  deep-seated  origin,  should  not  be  persistent,  it  is 
well  known  that  few  contact  deposits  are  mined  at  more  than 
shallow  depths. 

In  most  cases  certain  beds  of  the  intruded  rocks  exert  a  greater 
precipitative  action,  or  are  more  easily  replaced,  than  other  beds 
of  the  series,  and  contact  minerals  develop  much  more  abundantly 
in  them  than  elsewhere.  It  is  necessary  in  the  examination  of 
contact  deposits  to  trace  these  favorable  beds  and  to  determine 
their  probable  thickness  beneath  the  better  exposures;  not  infre- 
quently, the  important  mineralization  is  confined  to  one  or  more 
such  beds,  to  which,  therefore,  exploration  should  be  limited, 
and  below  which  the  mineralization  should  not  .be  expected  to 
extend. 

Most  contact  minerals  are  resistant  to  weathering,  and  erosion 
is  often  halted  at  the  horizon  where  the  contact  metamorphic 
silicates  reach  their  largest  development.  Large  outcrops  do 
not,  therefore,  afforc^a  reliable  criterion  of  the  extent  of  such 

1  Waldemar  Lindgren,  Trans.,  A.  I.  M.  E.,  Vol.  XXXI,  p.  227. 


TYPES  OF  PRIMARY  ORE-DEPOSITS  89 

deposits  in  depth.  A  large  development  of  contact  metamorphic 
silicates  does  not  necessarily  indicate  the  existence  of  valuable 
ore  any  more  than  a  barren  quartz  vein  indicates  the  presence  of 
gold.  Whether  or  not  such  an  outcrop  contained  sulphides 
before  oxidation  may  usually  be  determined  by  a  search  for  and 
examination  of  the  casts  left  in  the  resistant  silicates  upon  the 
solution  of  contained  sulphides;  this  feature  is  taken  up  in  the 
chapter  on  outcrops.  The  ores  of  contact  deposits,  unless  of 
smelting  grade,  offer  serious  metallurgical  difficulties,  owing  to 
the  high  specific  gravity  of  their  gangue  minerals. 

The  phenomena  of  contact  metamorphism  are  considered  in 
the  chapter  on  Primary  Alterations  of  Wall  Rocks. 

AT  MORENCI,  ARIZONA/  important  contact  deposits  occur  in 
limestones  and  shales  along  intrusions  of  quartz-monzonite  por- 
phyry. Wherever  the  porphyry  came  in  contact  with  granite  or 
quartzite,  little  alteration  is  observed,  but  wherever  the  porphyry 
meets  the  limestones  or  shales  of  the  Paleozoic  series  extensive 
contact  metamorphism  has  taken  place.  The  whole  Paleozoic 
series  is  affected,  but  more  particularly  the  pure  limestone  of  the 
lower  Carboniferous,  which,  for  a  distance  of  several  hundred  feet 
from  the  contact,  has  been  converted  into  an  almost  solid  mass  of 
garnet.  The  shales  have  suffered  less  from  this  metamorphism, 
but  near  the  porphyry  are  likely  to  contain  epidote  and  other 
accessions.  Wherever  alteration  by  surface  agencies  has  not 
masked  the  phenomena,  magnetite,  pyrite,  chalcopyrite,  molyb- 
denite, specularite  and  zincblende  accompany  in  various  pro- 
portions the  contact-metamorphic  minerals,  which  here  comprise 
garnet,  epidote,  diopside,  tremolite  and  quartz.  In  form,  the 
deposits  in  limestone  are  irregular,  but  in  many  cases  they  assume 
a  tabular  shape,  due  to  the  accumulation  of  the  minerals  along 
certain  favorable  planes  of  stratification,  or  along  the  walls  of 
dikes.  These  contact  deposits  differ  from  most  examples  in  the 
absence  of  wollastonite  and  vesuvianite,  and  in  the  metamorph- 
ism of  the  shales;  instead  of  the  knotty  schist  or  hornfels  usually 
produced  from  shales  there  is  found  at  Morenci  a  greenish  horn- 

1  Waldemar  Lindgren,  P.  P.  43,  U.  S.  G.  S.,  p.  19. 


90  EXAMINATION  OF  PROSPECTS 

fels,  with  much  amphibole  (tremolite),  epidote,  pyrite  and 
magnetite. 

Pegmatitic  Deposits. — The  characteristic  minerals  of  pegmatitic 
deposits1  are  magnetite,  bornite,  arsenopyrite,  molybdenite,  cassi- 
terite  and  wolframite,  associated  with  quartz,  muscovite,  alkali 
feldspars,  tourmaline,  apatite,  fluorite,  spodumene,  and  more 
rarely,  hornblende  and  soda-lime  feldspars.  These  deposits, 
which  are  of  deep-seated  origin,  contain  little  gold  and  silver  and 
except  where  associated  with  stockworks  and  cassiterite-bearing 
impregnations  they  are  irregular  in  value  and  of  slight  economic 
importance,  except  for  mica  and  minerals  of  the  rare  earths. 
Pegmatite  dikes  carrying  wolframite  are  known  at  many  places 
in  the  western  United  States;  pockets  of  rich  ore  are  occasionally 
found  in  them,  but  these  deposits  apparently  have  not  repaid 
exploration. 

Fahlbands. — Beds  of  schist  that  have  been  impregnated  with 
sulphides  and  subjected  to  dynamo-regional  metamorphism  are 
known  as  fahlbands.2  The  sulphides  occur  disseminated  through 
the  schist  intergrown  with  the  principal  minerals  of  the  rock,  and 
also  along  the  planes  of  schistosity;  the  association  is  such  as 
to  render  obscure  their  origin  and  mode  of  introduction.  These 
deposits,  which  are  rarely  of  economic  importance,  are  apparently 
confined  to  areas  of  pre-Cambrian  rocks.  Fahlbands  frequently 
persist  over  long  distances,  but  it  is  rare  that  their  mineralization 
is  sufficiently  concentrated  to  form  ore.  In  certain  European 
localities,  they  have  exerted  an  important  influence  upon  the 
segregation  of  values  in  veins  that  cross  them. 

IN  THE  UPPER  PECOS  DISTRICT,  NEW  MEXICO, 3  an  aniphibolite, 
probably  produced  by  regional  metamorphism  from  a  dioritic  or 
diabasic  rock,  carries  disseminated  chalcopyrite  and  zincblende 
that  contain  a  little  gold  and  silver,  associated  with  a  green  bio- 
tite,  tourmaline,  and  veinlets  of  dark  quartz,  and  intergrown 
with  the  principal  minerals  of  the  containing  rock. 

1  Waldemar  Lindgren,  Economic  Geology,  Vol.  II,  p.  111. 

2  J.  F.  Kemp,  "Ore  Deposits,"  p.  73. 

3  Lindgren,  Graton  and  Gordon,  P.  P.  68,  U.  S.  G.  S.,  p.  50. 


TYPES  OF  PRIMARY  ORE-DEPOSITS 


91 


Regionally  Metamorphosed  Ore -Deposits. — Deposits  that  were 
formed  in  remote  geological  ages  are  likely  to  have  been  deeply 
buried,  and  to  have  undergone  rearrangement  under  the  stresses 
of  dynamo-regional  metamorphism.  The  recrystallization  result- 


Schist  Ore  Zone  Gossan  Chalcocite  Ore 

FIG.  43.  —  Section  of  the  ore-deposit  in  the  Mary  mine,  Ducktown, 
Tennessee,  showing  a  regionally  metamorphosed  ore-deposit.  After 
Emmons  and  Laney. 


ing  from  these  processes  may  so  completely  change  the  enclosing 
rocks  to  crystalline  schists  as  to  hide  completely  their  original 
character,  it  being  difficult  to  distinguish  schists  that  result 
from  the  metamorphism  of  sedimentary  beds  from  those  that  were 
originally  igneous  rocks.  Under  these  conditions  of  pressure, 


92  EXAMINATION  OF  PROSPECTS 

heat  and  flowage  the  small  amount  of  moisture  contained  in  the 
rocks  appears  sufficient  to  accomplish  a  complete  rearrangement 
of  the  minerals.  Ore-deposits  contained  in  these  rocks  have 
undergone  complete  transformation,  and  their  original  relation- 
ships have  been  obscured  to  such  an  extent  that  their  origin  is  not 
determinable;  they  are,  therefore,  best  described  as  a  separate 
type.  Rearrangement  under  these  conditions  brings  about  a 
segregation  of  ore  minerals  into  homogeneous  bodies,  and  may  be 
considered  a  process  of  primary  concentration,  the  ores  resulting 
from  rearrangement  without  accession  of  material  from  without. 
Minerals  that  are  stable  under  the  conditions  of  dynamo-regional 
metamorphism  and  which  therefore  are  characteristic  of  these 
deposits  are:1  pyrite,  chalcopyrite,  pyrrhotite,  magnetite,  quartz, 
muscovite,  biotite,  epidote,  hornblende  and  albite. 

Regionally  metamorphosed  deposits  usually  occur  parallel  to 
the  schist osity  of  the  enclosing  rock,  or  cross  it  at  low  angles,  in 
accord  with  the  law  that  tabular  bodies  under  stress  and  flowage, 
tend  to  orient  themselves  in  the  direction  of  least  pressure.2  The 
relations  of  contact  deposits  to  igneous  rocks  are  likely  to  have 
been  destroyed  by  the  metamorphism,  and  the  fissures  and  chan- 
nels that  produced  replacement  deposits  are  likely  to  have  been 
healed.  The  usual  guides  in  exploration  for  these  deposits, 
therefore,  are  obliterated.  The  most  marked  structural  features 
of  regionally  metamorphosed  ore-deposits  are  their  lenticular 
form,  and  the  frequent  distribution  of  such  lenses  in  an  over- 
lapping series.  The  occurrence  of  these  primary  ore-bodies  is 
discussed  under  "Lenticular  Ore-Shoots  "  in  a  succeeding  chapter. 
The  individual  lenses  of  deposits  of  this  type  are  occasionally  of 
large  size,  but  not  infrequently  the  mineralization  is  confined  to 
a  single  lenticular  mass,  and  expectations  of  future  ore  are  not 
justified  beyond  the  probable  content  of  the  lenses  already 
exposed.  These  deposits,  being  thickest  at  their  centers  and 
narrowing  to  extinction  toward  their  peripheries,  often  permit 
approximate  estimates  of  their  content  to  be  made  in  advance  of 

1  Waldemar  Lindgren,  Economic  Geology,  Vol.  II,  p.  127. 

2  W.  H.  Emmons,  Economic  Geology,  Vol.  IV,  p.  776. 


TYPES  OF  PRIMARY  ORE-DEPOSITS  93 

exploration.  The  section  presented  by  the  surface  is  likely  to  be 
a  fair  criterion  of  the  distribution  of  deposits  of  this  type  in 
depth.  In  exploration  for  further  deposits  cross-cuts  should  be 
driven  near  the  extremities  of  the  known  lenses  to  expose  possible 
overlapping  bodies;  in  most  cases  little  other  exploration  is 
justified. 

THE  VERMONT  COPPER  BELT1  contains  three  districts — Corinth, 
Copperfield  and  South  Strafford.  The  deposits  occur  along  a 
due  north-south  line,  which  corresponds  to  the  general  direction 
of  the  schistosity  of  the  rocks.  The  rocks  are  micaceous  schists 
and  gneisses,  formed  from  sandstones  and  shales  by  regional 
metamorphism.  The  original  bedding,  though  obscure,  is 
occasionally  in  evidence,  and  does  not  correspond  with  the  folia- 
tion. Intrusions  of  granite  are  common  in  the  region,  but  do  not 
occur  in  the  immediate  vicinity  of  the  ore-deposits.  The  ore 
bodies  are  lenticular  masses  which  simulate  bedded  deposits, 
since  they  appear  to  conform  to  the  banding  of  the  enclosing 
schists.  At  each  locality  only  one  workable  lens  has  been  found 
outcropping;  in  the  deep  mines  the  outcropping  lens  wedges  out 
in  depth,  but  is  found  to  overlap  the  tapered  upper  end  of  another 
lens  in  the  foot-wall.  The  deposits  have  no  gossan  cap,  sulphides 
appearing  at  the  surface.  The  ores  consist  of  massive  pyrrhotite, 
chalcopyrite,  pyrite,  and  a  little  sphalerite  mixed  with  variable 
quantities  of  quartz  and  actinolite;  in  the  leaner  ores,  garnet  and 
biotite  are  present. 

Deposits  Due  to  the  Filling  of  Open  Spaces. — The  filling  of 
open  fissures  or  preexisting  cavities  in  rocks  is  a  process  of  great 
importance  in  ore  deposition;  while  many  veins  are  due  to  this 
process  alone,  a  majority  of  mineral  veins  are  probably  the 
result  of  both  replacement  and  the  filling  of  open  spaces.  Open 
spaces  are  caused  by  irregular  fissuring  accompanied  by  a 
moderate  movement,  sufficient  to  bring  projection  opposite 
projection  and  concavity  opposite  concavity,  and  thus  cause 
pinches  and  swells  in  the  fissure  and  resulting  vein. 

It  is  clear  that  a  uniform  open  space  of  large  extent  cannot 

1  W.  H.  Weed,  Bull.  225,  U.  S.  G.  S.,  p.  199. 


94 


EXAMINATION  OF  PROSPECTS 


remain  open  along  a  fissure  for  any  length  of  time,  but  must 
soon  be  closed  by  the  pressure  of  overlying  rocks.  Fissures, 
therefore,  that  remained  open  for  sufficient  lengths  of  time  to 
become  filled  and  mineralized  are  commonly  irregular  in  cross- 
section,  the  open  stretches  being  separated  by  tight  portions 
where  the  wall  rocks  came  together  and  formed  the  buttresses 
that  permitted  intervening  portions  to  remain  open.  It  is  a 
common  fallacy  that  filled  fissures  are  characterized  by  uni- 
formity of  direction  and  thickness;  in  most  veins,  which  are  the 
result  of  both  filling  and  replacement,  the  latter  process  tends 
to  counteract  and  to  obscure  the  irregularities  of  the  orignial 
open  spaces. 

a        b  c    d  e  d'  e  b  a 


FIG.  44. — Section  of  a  crustified  vein  near  the  London  shaft,  Silverton, 
Colorado,  a,  Country  rock;  b,  quartz  and  cholcopyrite;  c,  tetrahedrite; 
d-df,  quartz;  e,  galena.  After  Ransome. 

Enlargement  by  solution  is  thought  by  many  to  play  a  large 
part  in  the  formation  of  cavities  that  are  subsequently  filled 
with  ore.  Except  in  limestone,  perhaps,  such  solution  takes 
place  contemporaneously  with  and  constitutes  a  part  of  the 
process  of  replacement. 

Open  fissures  and  cavities  in  rocks  are  more  abundant  near  the 
surface  than  in  depth,  because  of  the  less  pressure  of  overlying 
rocks  that  tends  to  close  them.  While  a  majority  of  deep-seated 
veins  are  replacement  veins,  open  cavities  frequently  persist  to 


T-YPES  OF  PRIMARY  ORE-DEPOSITS  95 

great  depths,  especially  in  veins  that  pinch  and  swell  as  before 
described. 

According  to  C.  R.  Van  Hise1  the  zone  of  flowage  from  pres- 
sure of  overlying  rocks,  in  which  no  cavities  can  exist,  is  reached 
at  1625  ft.  in  soft  shales  and  at  32,500  ft.  in  firm  granites. 

Fissuring  accompanied  by  sufficient  movement  to  form  open 
spaces  in  one  rock,  may  produce  a  tight  fissure  in  a  more 
plastic  rock  at  the  same  depth,  and  veins  that  are  the  result  of 
the  filling  of  open  spaces  may  be  expected  to  vary  in  width 
according  to  the  character  of  the  rock  traversed;  a  vein  that 
cuts  a  series  of  different  beds  may  be  expected  to  vary  markedly 
in  passing  through  them. 

If  the  valuable  mineral  in  a  filled  fissure  was  among  the 
earliest  deposited,  it  will  be  found  near  the  walls,  and  may  be 
expected  to  persist  longitudinally  along  the  vein;  if  it  was  among 
the  last  deposited,  it  is  likely  to  be  limited  to  the  spaces  at  the 
center  that  remained  open  at  the  time  of  its  deposition,  and, 
therefore,  to  be  more  markedly  confined  to  shoots. 

The  usual  criterion  of  a  filled  fissure  is  the  arrangement  of  the 
filling  in  crusts  or  bands  parallel  to  the  walls,  similar  bands 
occupying  the  same  relative  positions  on  either  side  of  the  center 
line,  which  is  frequently  marked  by  interlocking  combs  of  crystals. 
The  bands  nearest  the  walls  represent  the  minerals  first  deposited, 
and  the  central  part  the  latest  deposition.  In  cases  where  the 
vein  after  being  filled  has  been  reopened,  the  additional  crusts 
deposited  will  not  be  symmetrical  with  respect  to  the  original 
arrangement,  and  occasionally  veins  are  seen  that  in  this  way 
exhibit  a  record  of  repeated  opening  and  filling. 

While  in  filled  deposits  evidence  of  crustification  is  often 
visible,  not  infrequently  the  filling  is  homogeneous  or  quite 
irregular,  and  the  method  of  vein  filling  is  not  apparent,  unless 
disclosed  by  the  examination  of  thin  sections  under  the  micro- 
scope. The  presence  of  radiating  clusters  of  crystals  is  indica- 
tive of  deposition  in  an  open  space,  as  is  also  a  lack  of  alteration 
of  included  fragments  of  the  wall  rocks.  Not  infrequently, 

1  Sixteenth  Annual  Report  U.  S.  G.  S.,  I,  p.  312. 


96  EXAMINATION  OF  PROSPECTS  • 

replacement  veins  exhibit  a  banded  structure;  this  is  the  result 
of  a  thin  sheeting  of  the  country  rock,  the  lines  of  which  are 
repeated  and  preserved  in  the  arrangement  of  the  vein  minerals, 
the  replacement  having  taken  place  along  the  sheeting  planes 
first,  and  later,  under  different  conditions,  having  penetrated 
the  intervening  slabs.  A  reopening  of  a  fissure  during  or  after 
mineralization  may  give  rise  to  a  structure  in  close  imitation  of 
crustification. 

The  lines  of  demarkation  between  ore  and  wall  rock  are 
commonly  well-defined  in  filled  deposits,  while  in  replacement 
deposits  the  ore  is  likely  to  merge  gradually  into  the  walls.  That 
the  action  of  the  solutions  was  confined  to  the  open  spaces  in 
filled  fissures  is  not  invariably  the  rule;  frequently  the  ore  is 
found  along  a  well-defined  wall,  beyond  which  the  ore  minerals 
do  not  penetrate,  but  the  rock  bordening  the  vein  is  altered, 
evidently  by  the  action  of  the  vein-forming  solutions.  It  is 
probable  in  such  cases  that  the  walls  of  the  vein  acted  as  dialyzers, 
restraining  the  passage  of  certain  elements  and  compounds, 
which  precipitated  within  the  vein,  but  permitting  the  passage 
of  the  solutions  that  altered  the  adjacent  rocks. 

AT  PINOS  ALTO'S,  NEW  MEXICO/  the  process  of  open  fissure  fill- 
ing is  well  illustrated  in  the  Pacific  vein.  Five  distinct  bands 
may  be  counted.  Proceeding  from  each  wall  inward  to  the  center, 
each  of  these  bands  has  an  almost  perfect  counterpart  on  the 
opposite  side  of  the  vein.  The  first  band,  that  next  to  the  wall, 
contains  quartz  and  pyrite;  its  inner  edge  is  outlined  by  the 
crystalline  terminations  of  quartz  prisms,  a  beautiful  example  of 
comb  structure.  The  succeeding  band  is  composed  of  zinc- 
blende  and  chalcopyrite;  the  chalcoprite  grows  more  abundant 
toward  the  inner  edge  and,  in  fact,  forms  two  subsidiary  bands 
separated  by  a  thin  band  of  sphalerite.  The  next  layer,  a  thin 
band,  contains  quartz  and  chalcopyrite.  It  is  followed  by  a 
narrow  band  of  sphalerite,  which  in  turn  is  followed  by  a  thicker 
band  of  quartz  that  contains  fine  grains  of  disseminated  chalco- 
pyrite, and  locally  fails  to  join  with  its  corresponding  band  on 

1  Sidney  Paige,  Bull.  470,  U.  S.  G.  S.,  p.  114. 


TYPES  OF  PRIMARY  ORE-DEPOSITS 


97 


the  opposite  side,   leaving   an  open   crystalline   cavity   at  the 
center. 

On  one  wall  of  the  vein  is  a  narrow  secondary  vein,  evidently 
a  reopened  fissure.  Its  walls  are  outlined  by  narrow  bands  of 
quartz  (with  a  little  chalcopyrite  and  galena),  between  which 
is  a  pinkish  cream-colored  mass  of  iron  and  magnesia  carbon- 


a  b  c  d  e  f 

FIG.  45. — Specimen  from  the  Pacific  vein  near  Pinos  Altos,  New  Mexico, 
showing  crustification.  a,  Chalcopyrite;  6,  pyrite;  c,  zincblende;  d,  quartz; 
e,  sericitized  porphyry;/,  carbonates,  quartz,  and  iron  oxide.  After  Paige. 

ates,  and  quartz.  The  narrow  quartz  bands  forming  the  walls 
of  this  little  vein  have  locally  been  broken,  and  pieces  of  the 
wall  now  lie  at  varying  angles  across  the  vein,  embedded  in  the 
vein  filling. 

On  the  opposite  wall  of  the  main  vein  a  fragment  of  country 
rock  is  included  in  and  surrounded  by  the  vein  material  of  the 
large  vein. 

7 


98  EXAMINATION  OF  PROSPECTS 

From  this  data  the  history  of  the  mineralization  of  this  partic- 
ular vein  may  be  deduced.  A  fracture  was  formed  in  the  country 
rock  and  filled  by  solutions  carrying  zinc  and  iron  sulphides. 
Fracturing  continued  and  cross  fissures  on  a  small  scale  were 
opened.  The  forces,  of  whose  presence  this  first  fracturing  was 
a  preliminary,  finally  succeeded  in  producing  an  open  fracture 
measured  by  the  width  of  the  vein  described,  and  solutions 
carrying  silica,  iron  sulphide,  a  trace  of  zinc  sulphide,  and  lead, 
circulated  through  the  open  spaces  thus  afforded.  Along  both 
walls  quartz  and  pyrite  were  precipitated  simultaneously,  and 
continued  to  be  precipitated,  apparently,  until  solutions  ceased 
to  circulate,  or  ceased  to  carry  sulphur,  iron  and  silica,  for  the 
boundary  between  the  first  band  and  the  succeeding  one  is 
sharp  both  in  demarkation  and  in  mineral  content.  When 
mineralizing  waters  next  flowed  past  the  walls,  zincblende  and 
chalcopyrite  were  deposited,  and  it  is  evident  that,  although 
copper,  sulphur  and  iron  were  present  during  the  remainder  of 
the  history  of  the  vein,  though  growing  markedly  less  toward  the 
end,  the  zincblende  and  silica  content  fluctuated,  first  a  layer  of 
one  and  then  of  the  other  being  precipitated.  Parts  of  the  vein 
along  the  center  were  probably  never  completely  filled,  not  be- 
cause there  was  a  lack  of  material,  but  because  deposition  fortui- 
tously isolated  geode-like  open  spaces  within  which  the  circu- 
lation ceased.  The  small  vein  at  the  edge  of  the  large  one  points 
to  a  recurrence  of  fracturing,  and  the  advent  of  carbonated 
waters,  carrying  silica  also;  it  marks  a  distinct  change  in  the 
solutions,  with  the  cessation  of  which  the  mineralization  closed. 

Replacement  Veins. — A  majority  of  veins  are  in  part,  at  least, 
the  result  of  the  replacement  of  their  walls  by  mineralizing  solu- 
tions, and  in  many  cases  the  process  of  fissure  filling  was  probably 
so  subordinate  as  to  be  practically  negligable;  the  original  fissures 
of  replacement  veins,  which  were  probably  narrow,  acted  chiefly 
as  channels  for  the  passage  of  the  replacing  and  mineralizing 
solutions.  While  in  a  filled  fissure  the  width  of  the  vein  repre- 
sents the  width  of  the  cavity  filled,  and  the  line  of  demarkation 
between  ore  and  wall  rock  is  sharp,  in  a  replacement  vein  the 


TYPES  OF  PRIMARY  ORE-DEPOSITS 


99 


width  bears  no  relation  to  the  size  of  the  original  fissure,  and  the 
line  of  demarkation  between  ore  and  wall  rock  is  commonly  ill- 
defined. 

Replacement  veins  vary  greatly  in  size  according  to  the  ease 
of  solubility  or  replacement  of  the  rock  traversed,  and  their 
values  are  likely  to  be  localized  by  the  chemical  precipitative 
action  of  the  different  rocks  traversed.  Replacement  veins  are 
commonly  accompanied  by  metasomatic  alteration  of  their  walls 
as  will  be  described  in  a  succeeding  chapter;  fragments  of  the 


Black  shale 


Quartzite 


)}.:.£'     Gray  sandstone 


FIG.  46. — Type   of  silver  bearing  vein  modified   by   replacement,  Ouray, 
Colorado,  showing  the  varying  size  in  different  rocks.     After  Irving. 

wall  rocks  included  in  replacement  veins  are  likely  to  be  partly 
or  completely  replaced  by  ore;  the  original  outlines  of  such 
fragments  are  usually  preserved  in  the  arrangement  of  the 
replacing  minerals,  which  may  enclose  cores  of  fresh  or  partly 
altered  rock,  and  thus  conclusively  prove  the  process  of 
replacement. 

Replacement  Deposits. — Metasomatic  replacement  of  the 
country  rock  by  mineralizing  solutions  is  a  process  of  great 
importance  in  ore  deposition,  and  there  are  probably  few  epige- 


100  EXAMINATION  OF  PROSPECTS 

netic  deposits  in  the  formation  of  which  it  has  not  played  some 
part.  In  a  classification  of  ore  deposits  for  the  purpose  of  study 
and  practical  correlation  it  appears  best  to  discuss  under  this 
head  those  replacement  deposits  that  are  relatively  homogeneous, 
taking  up  the  other  types  according  to  their  most  prominent 
characteristics. 

Replacement  deposits  are  most  common  in  the  more  easily 
soluble  and  replaceable  rocks,  among  which  limestone  is  most 
prominent.  That  the  precipitative  action  of  the  replaced  rock 


FIG.  47. — Replacement  of  limestone  by  copper  ore,  Bingham,  Utah,  show- 
ing the  greater  replacement  of  selected  beds.     After  BoutwelL 

is  not  the  controlling  feature  of  the  process  is  shown  by  the 
occurrence  of  large  replacement  deposits  in  quartzite,  shales, 
schists  and  other  rocks  as  well  as  in  limestones.  The  relative 
replaceability  of  a  rock  varies,  of  course,  with  the  chemical 
composition  of  the  mineralizing  solution,  certain  solutions  attack- 
ing limestone  with  greatest  activity,  while  others  replace  quartzite 
with  equal  ease;  in  a  given  deposit,  however,  a  rock  that  has  been 
extensively  replaced  must  be  considered  the  ore-bearing  rock,  as 
as  it  is  unlikely  that  a  different  rock  will  have  yielded  equally 


TYPES  OF  PRIMARY-  QRV-DFPO$1TS 


101 


to  the  attack  of  the  same  mineralizing  solutions;  important 
deposits  in  a  bed  of  limestone,  for  example,  may  not  be  expected 
to  continue  into  underlying  quartzite  or  granite.  It  often 
happens  that  one  bed  only  of  a  series  of  apparently  similar  rocks 
has  offered  a  favorable  horizon  for  ore  deposition,  other  beds 


N  W 


feet 


FIG.  48. — Replacement  of  limestone  by  argentiferous  galena,    Bingham, 
Utah.     After  Boutwell. 

being  lean  or  barren.     In  such  cases  a  study  of  the  stratigraphy 
is  imperative. 

Replacement   deposits   commonly   occur   in  the   vicinity   of, 
although  not  adjacent  to,  intrusive  rocks,  and  are  in  some  cases 


102 


EXAMINATION  OF  PROSPECTS 


closely  related  to  contact  deposits;  in  many  instances,  however, 
replacement  deposits  occur  without  visible  association  with 
intrusives,  the  mineralizing  solutions  having  gained  access  to  the 
replaced  beds  or  rocks  through  fissures.  Replacement  deposits 
unless  occurring  along  a  prominent  fissure,  are  likely  to  be  as 
irregular  in  distribution  through  the  mineralized  horizon  as  they 
are  individually  in  form.  Replacement  deposits  are  frequently 


FIG.  49. — Section  through  the  ore-bodies  at  Sierra  Mojada,  Coahuila, 
Mexico,  showing  irregular  replacement  deposits  in  limestone.  After 
Malcolmson. 

connected  with  each- other  or  with  the  main  circulation  channel, 
by  feeders,  or  veinlets,  which  often  are  quite  barren  of  minerals 
and  with  difficulty  traceable.  Such  obscure  feeders  are  often 
the  only  guides  in  exploration  for  new  deposits. 

The  degree  of  shattering,  brecciation  or  straining  is,  next  to 
the  replaceability  of  the  rock,  the  most  important  factor  in 
determining  the  position  of  replacement  deposits;  the  greater 


TYPES  OF  PRIMARY  ORE-DEPOSITS 


103 


the  area  of  the  surfaces  exposed  in  relation  to  the  mass  of  rock, 
the  more  rapidly  and  thoroughly  does  replacement  take  place, 
and  the  existence  of  these  deposits  may  occasionally  be  pre- 
dicted through  tracing  the  shattered  or  strained  zones.  The 
passage  of  solutions  through  rocks  is  seen  under  the  microscope 
to  take  place  even  where  the  existence  of  cracks  or  minute 
fissures  is  not  discernable,  and  the  invading  mineral  may  form 
grains  that  are  apparently  completely  surrounded  by  fresh,  solid 
rock;  a  zone,  therefore,  within  which  the  rock  has  been  subjected 


FIG.  50. — Sections  showing  mineralizing  fissures  and  replacement  of 
limestone  'strata  by  siliceous  gold  ore,  Black  Hills,  South  Dakota.  After 
Irving. 

to  slight  straining  of  the  particles  only,  without  rupture,  may 
afford  access  to  the  replacing  solutions.  Shattering  is  a  less 
evident  factor  in  the  formation  of  replacement  deposits  in  lime- 
stone than  in  other  rocks,  probably  because  of  its  ready  solubility. 

Replacement  deposits  are  rarely  of  great  vertical  extent,  and 
very  often  the  rock  above  them  affords  no  indication  of  their 
existence.  Outcrops  of  these  deposits,  therefore,  are  accidental, 
and  relatively  scarce,  and  the  existence  of  one  such  body  having 
been  established,  blind  exploration  for  new  deposits  in  the  same 
horizon  is  more  often  justified  than  is  the  case  with  deposits  of 
other  types. 

The  boundaries  of  replacement  deposits   are  usually  poorly 


104  EXAMINATION  OF  PROSPECTS 

defined,  the  ore  merging  gradually  into  the  enclosing  rock, 
where  the  boundary  between  ore  and  waste  becomes  a  question 
of  assay  only.  Occasionally,  one  mineral  has  penetrated  the  rock 
more  easily  than  the  others,  and  so  preponderates  in  the  outer 
parts  of  the  deposits;  in  exploration,  therefore,  upon  meeting 
such  a  lean,  or  perhaps  barren,  mineralization,  it  may  indicate 
the  existence  of  payable  ore  beyond. 

Replacement  ores  frequently  reproduce  the  structure  of  the 
replaced  rock,  certain  bedding  planes  being  followed  in  preference 
to  others,  and  where  this  condition  obtains  the  bedding  planes 
are  usually  parallel  to  the  greatest  dimension  of  the  ore-body. 

The  principal  alterations  of  limestones  as  an  accompaniment  of 
mineralization  are  silicification,  marmorization,  and  to  a  less 
extent,  the  development  of  a  disseminated  pyrite  mineralization, 
any  of  which  may  afford  clews  to  the  existence  of  ore-bodies.  The 
primary  alterations  that  accompany  mineralization  will  be 
discussed  in  a  later  chapter. 

IN  THE  COEUR  D'ALENES,  IDAHO/  large  replacement  deposits 
have  formed  along  shattered  zones  in  quartzite.  In  the  Bunker 
Hill  mine  the  ore  consists  of  galena  and  siderite,  with  small 
quantities  of  quartz,  zincblende  and  pyrite,  the  gangue  being  the 
enclosing  quartzite.  It  appears  that  siderite  first  replaced  the 
quartzite,  and  that  later  argentiferous  galena  partially  replaced 
the  siderite,  though  the  direct  replacement  of  quartzite  by  galena 
is  occasionally  noted.  The  best  ore  consists  of  rather  fine- 
grained masses  of  galena  with  subordinate  siderite,  which  grades 
into  ore  in  which  the  siderite  exceeds  the  galena,  and  this  into 
barren  quartzite.  The  ore  is  principally  a  replacement  of  the 
Revett  quartzite,  but  the  replacement  is  closely  connected  with 
fissuring,  and  some  of  the  galena  was  deposited  in  open  spaces. 
In  some  of  the  important  stopes,  quartz  and  pyrite  are  usually 
most  conspicuous  in  the  transition  zone  from  ore  to  country  rock. 
The  zone  of  fissured  quartzite  in  which  the  ore-bodies  occur  has 
a  maximum  width  of  300  ft.  measured  perpendicularly  to  the 
Bunker  Hill  fissure.  Within  this  zone,  here  in  contact  with  the 

1  F.  L.  Ransome,  P.  P.  62,  U.  S.  G.  S.,  p.  162. 


TYPES  OF  PRIMARY  ORE-DEPOSITS  105 

foot- wall,  there  separated  from  it  by  barren  quartzite,  are  numer- 
ous irregular  ore-bodies,  usually  without  definite  walls  or  bound- 
aries. Individual  ore-shoots  reach  500  ft.  in  length,  100  ft.  or 
more  in  width,  and  300  to  400  ft.  in  depth.  The  whole  fissured 
zone  may,  in  a  broad  sense,  be  regarded  as  a  single  great  lode, 
within  which  the  partly  overlapping  and  partly  connected  ore 
bodies  are  not  uniformly  distributed,  but  are  grouped  in  at  least 
four  fairly  distinct  shoots.  That  no  ore  should  have  been  de- 
posited beneath  the  persistent  seam  of  dark  gouge  characteristic 
of  this  fissure  is  remarkable,  as  the  quartzites  of  the  foot-wall, 
which  have  been  well  explored,  are  identical  in  character  with 
those  of  the  hanging  wall,  and  are  in  places  extensively  fissured 
and  broken,  though  usually  to  a  less  degree  than  in  the  hanging 
wall. 

Ix  THE  HIGHLAND  BOY  MINE,  BINGHAM,  UTAH/  important  re- 
placement deposits  of  copper  ore  occur  in  limestone.  The 
limestone  is  commonly  a  coarsely  crystalline  marble,  in  general 
more  cherty  toward  the  base  where  it  rests  upon  quartzite,  more 
massive  and  crystalline  above,  and  locally  siliceous.  The  ore 
bodies  lie  within  the  main  body  of  the  limestone  well  above  the 
underlying  quartzite  along  a  zone  of  fissuring  and  mineralization. 
Localization  of  ore  has  resulted  in  the  formation  of  three  well- 
defined  lenses  or  shoots,  the  largest  of  which  reaches  a  width  of 
400  ft.  and  a  thickness  of  100  ft.;  the  shoots  are  approximately 
conformable  to  the  bedding.  In  the  few  instances  where  cross- 
cutting  has  exposed  the  quartzite,  the  ore  does  not  make  down  to 
it.  The  walls  of  the  ore-bodies  are  commonly  slip  planes  or  beds 
of  siliceous  or  crystalline  limestone.  In  some  instances  the  upper 
parts  of  the  ore-bodies  become  progressively  leaner  until  they 
pass  into  the  barren  limestone  that  forms  the  hanging-wall; 
laterally,  the  ore  bodies  pinch  to  thin,  irregular  seams.  The 
ore  consists  of  pyrite,  chalcopyrite  with  some  bornite  and  chalco- 
cite  (secondary?)  associated  with  small  quantities  of  galena, 
specularite,  marcasite,  enargite  and  zincblende;  galena  is  practi- 
cally restricted  to  fracture  zones.  The  ore  carries  small  but 

1  J.  M.  Boutwell,  P.  P.  38,  U.  S.  G.  S.,  p.  267. 


106  EXAMINATION  OF  PROSPECTS 

commercially  valuable  quantities  of  gold  and  silver.  In  the 
exploration  for  ore-bodies  in  this  district  (Ibid.,  p.  154)  on 
approaching  a  shoot  of  copper  sulphide  ore  that  lies  within  barren 
marble,  lean  ore  may  be  observed  in  certain  beds.  This  grad- 
ually becomes  larger  in  proportion  to  the  barren  rock,  and  higher 
in  grade,  until  the  entire  bed  or  beds  are  ore.  In  the  extreme 
outer  parts  of  these  shoots  bands  of  country  rock  alternate  with 
bands  of  lean  ore,  and  within  the  shoot  the  original  bedded 
character  is  sometimes  preserved  by  bands  of  barren  siliceous 
material,  and  in  some  cases  the  massive  ore  itself  preserves  the 
bedded  structure. 

IN  SHASTA  COUNTY,  CALIFORNIA/  large  masses  of  pyritic  copper 
ore  have  formed  as  replacement  deposits  in  alaskite  porphyry 
and  also  occasionally  extend  into  shale;  the  largest  of  these 
deposits,  the  Iron  Mountain,  probably  originally  contained 
20,000,000  tons  of  pyritic  ore.  The  ore-bodies  are,  in  general, 
roughly  tabular,  and  although  irregular  in  form,  may  best  be 
referred  to  as  lenses.  The  ores,  which  consist  of  pyrite,  chal- 
copyrite,  and  zincblende  with  subordinate  galena  associated  with 
quartz,  calcite  and  barite,  chlorite  and  sericite,  commonly  merge 
gradually  into  the  surrounding  rock.  The  ore-bodies  occur  in 
zones  of  highly  shattered  and  comminuted  rock,  and  this  con- 
dition is  apparently  the  determining  factor  in  their  localization. 
That  the  chemical  composition  of  the  enclosing  rock  was  not 
the  controlling  factor  in  deposition  is  proved  by  the  replacement 
in  different  deposits  of  alaskite,  shale,  and  of  a  basic  dike. 

IN  THE  BLACK  HILLS,  SOUTH  DAKOTA,2  in  the  Bald  Mountain 
District,  large  replacement  deposits  of  siliceous  gold  ores  have 
formed  in  limestone.  Long,  narrow,  restricted  fissures  exhibit- 
ing, in  general,  a  common  trend,  pass  upward  through  a  series  of 
shales,  limestones  and  quartzites.  Where  these  fissures  intersect 
the  limestone,  the  ore,  consisting  of  pyrite  and  quartz  carrying 
gold  and  silver,  replaces  the  rock  for  considerable  distances  either 
side  of  the  mineralizing  fissure;  where  many  fissures  are  grouped 

1  L.  C.  Graton,  Bull.  430,  U.  S.  G.  S.,  p.  89. 

2  J.  D.  Irving,  Economic  Geology,  Vol.  Ill,  p.  149. 


TYPES  OF  PRIMARY  ORE-DEPOSITS  107 

together  the  mineralization  from  them  has  coalesced  to  form 
flat  masses  of  great  lateral  extent.  The  mineralization  along  the 
fissures  themselves  is  commonly  slight,  and  is  often  absent,  and 
the  fissures  are  so  small  as  to  be  detected  with  difficulty  in  many 
instances. 

AT  SANTA  EULALIA,  CHIHUAHUA,  MEXICO/  the  ore-deposits 
form  great  masses  of  irregular  form  in  a  limestone  dome,  following, 
and  in  part  limited  by,  the  stratification  planes.  The  ore-bodies 
are  in  some  instances  connected  by  fissures,  by  films  of  red  clay,  or 
by  limestone  checked  and  netted  with  minute  fractures  filled  with 
iron  oxide.  The  ores  are  for  the  greater  part  oxidized,  and  con- 
sist of  more  or  less  impure  cerussite,  sometimes  containing  cores 
of  residual  galena.  The  replacement  of  the  limestone  is  indicated 
by  lines  of  chert  through  the  ore  that  correspond  to  similar  lines 
in  the  unaltered  limestone  walls,  and  by  the  presence  of  silicified 
fossils  in  the  ore.  The  district  is  one  of  the  most  important  in 
Mexico. 

Disseminated  Mineralizations. — An  important  class  of  ore- 
deposits  is  that  in  which  the  valuable  minerals  occur  as  minute 
particles,  or  narrow  seamlets,  or  stringers,  throughout  a  large 
mass  of  enclosing  country  rock.  The  number  of  such  mineraliza- 
tions whose  primary  ore  is  of  payable  grade  is  probably  small, 
but  these  deposits,  especially  those  that  contain  copper,  are  of 
the  greatest  importance  where  enriched  by  secondary  processes. 
Disseminated  mineralizations  are  probably  due  in  great  part  to 
metasomatic  replacement,  but  the  occurrence  of  their  minerals 
as  sparse  disseminations,  or  impregnations,  which  are  structural 
terms,  is  sufficient  to  warrant  their  description  as  a  separate 
type. 

Disseminated  mineralizations  are  most  frequent  in  schists  and 
in  intrusives ;  these  rocks  appear  to  favor  disseminated  minerali- 
zations in  much  the  same  way  that  limestone  appears  to  induce 
segregation  and  localization  of  introduced  minerals.  A  char- 
acteristic feature  of  disseminated  deposits  is  the  occurrence 
of  the  most  thorough  mineralization  in  the  areas  most  fissured 

1  W.  H.  Weed,  "Nature  of  Ore  Deposits,"  Beck- Weed,  p.  573. 


108  EXAMINATION  OF  PROSPECTS 

or  shattered.  In  many  cases  the  mineralization  accompanies 
reticulated  quartz  veinlets  through  the  shattered  rock,  and 
often,  especially  where  the  dissemination  occurs  in  the  mineral- 
izing intrusive,  the  introduced  minerals  are  abundant  along 
joint  planes  as  well  as  fracture  planes,  and  occur  in  much  less 
quantity  in  the  interior  of  the  masses  bounded  by  such  surfaces. 
Disseminated  mineralizations  are  commonly  closely  associated 
with  intrusives,  and  where  important,  extend  through  very 
large  rock  masses. 

AT  BINGHAM  CANYON,  UTAH/  a  great  mass  of  intrusive  monzon- 
ite  carries  throughout  an  irregular  but  persistent  mineralization 
of  finely  disseminated  pyrite  and  chalcopyrite  that  carry  low 
values  in  gold.  In  the  fresh  monzonite  these  minerals  occur  as 
minute  grains  scattered  through  the  rock,  and  along  joint  planes, 
and  also  embedded  in  irregular  quartz  veinlets.  A  correlation 
of  assays  with  structure  indicates  that  the  values  are  highest 
where  the  fissuring  and  veining  are  most  pronounced.  Through 
secondary  enrichment  this  deposit  has  yielded  copper-deposits 
of  the  first  rank. 

AT  CLIFTON,  ARIZONA, 2  intrusive  monzonite-porphyry  carries  a 
disseminated  mineralization  of  pyrite  and  chalcopyrite,with 
subordinate  zincblende  and  molybdenite,  associated  with  quartz 
and  a  sericitization  of  the  containing  rock.  This  mineraliza- 
tion is  most  intense  in  and  along  certain  veins  through  the 
monzonite-porphyry,  but  extends  as  impregnations  through 
the  rock,  along  joint  planes,  and  associated  with  quartz  veinlets, 
for  long  distances  either  side  of  the  mineralizing  veins.  The 
containing  rock  is  here,  also,  the  mineralizing  intrusive.  Large 
areas  of  the  intrusive  carry  sulphides,  but  the  mineralization  is 
apparently  most  intense  in  the  vicinity  of  the  centers  of  intrusion 
at  Morenci  and  near  Metcalf .  The  primary  ore  is  unpayable,  but 
through  secondary  enrichment  it  has  yielded  important  deposits. 

IN  THE  BURRO  MOUNTAINS,  NEW  MEXICO,  an  intrusion  of  mon- 
zonitic  porphyry  through  granite  has  been  accompanied  by  a  dis- 

1  J.  M.  Boutwell,  P.  P.  38,  U.  S.  G.  S.,  p.  259. 

2  Waldemar  Lindgren,  P.  P.  43,  U.  S.  G.  S.,  p.  202,  222. 


TYPES  OF  PRIMARY  ORE-DEPOSITS  109 

seminated  mineralization  consisting  of  minute  grains  and  seam- 
lets  of  pyrite  and  chalcopyrite  associated  with  quartz.  The 
mineralization  is  most  intense  along  certain  fracture  zones,  and 
appears  to  be  proportional  to  the  amount  of  shattering  of  the 
enclosing  rock.  The  primary  mineralization  is  unpayable,  but 
has  at  two  localities  yielded  important  deposits  through  second- 
ary enrichment. 

AT  THE  HOPEFUL  MINE,  NOGAL  DISTRICT,  NEW  MEXICO,  a  much 
altered  rock,  probably  sericitized  and  later  kaolinized,  carries  a 
disseminated  pyritic  mineralization  containing  gold  but  no 
copper.  The  pyrite  occurs  as  grains  through  the  rock,  which 
L.  C.  Graton  states  to  be  probably  a  monzonite,  and  is  most 
abundant  along  joint  planes  and  certain  ill-defined  fissures.  The 
gold,  which  is  stated  to  vary  between  $1.00  and  $3.50  per 
ton,  is  said  to  be  uniformly  distributed,  and  to  be  approxi- 
mately equal  in  the  oxidized  and  in  the  sulphide  ore. 

Conglomerate  Beds. — While  not  a  numerically  important 
type,  mineralized  beds  of  conglomerate  form  the  ores  of  two  of 
the  most  important  mining  districts  in  the  world.  The  origin 
of  these  deposits  is  not  clear,  and  while  to  a  certain  extent  similar, 
they  posses  features  that  render  difficult  their  classification 
with  the  more  common  and  better  understood  deposits.  In  both 
the  Michigan  copper  deposits  and  the  gold  deposits  of  the  Rand 
the  ore  consists  of  native  metals  in  the  cementing  material  of 
conglomerate  beds,  and  in  both  districts  the  mineralizations  are 
remarkably  persistent  over  great  areas,  and  to  great  depths. 

IN  THE  WITWATERSRAND,  SOUTH  AFRICA/  beds  or  "reefs"  of 
conglomerate  are  intercalated  with  quartzitic  sandstones  and, 
more  rarely,  slates;  there  are  eight  groups  of  these  reefs,  certain 
of  which  have  been  exploited  over  a  length  of  48  miles.  In 
thickness  the  reefs  vary  between  a  maximum  of  several  meters 
to  a  complete  wedging  out.  The  thinner  reefs  are  commonly  the 
richer.  The  intervening  strata  are  practically  barren,  although 
exceptions  to  this  rule  are  known.  The  reefs  are  composed 
of  pebbles,  commonly  ranging  from  a  hazel-nut  to  a  hen's  egg  in 

1  Beck-weed,  "Nature  of  Ore  Deposits,"  p.  512. 


110  EXAMINATION  OF  PROSPECTS 

size,  of  quartz  and  quartzite,  more  rarely  of  siliceous  schist, 
and  occasional  rounded  pyrite  granules;  the  pebbles  are  often 
deformed,  being  flattened  or  splintered,  and  themselves  rarely 
carry  any  mineralization.  The  cementing  material  is  composed 
of  small  quartz  granules  and  pyrite  with  associated  particles  of 
gold.  The  pyrite  occurs  in  rather  irregular  distribution,  and 
frequently  forms  crusts  around  the  pebbles  of  the  conglomerate, 
and  occasionally  is  concentrated  in  delicate  films  parallel  to  the 
stratification.  The  mineralization,  while  exhibiting  local  irregu- 
larity, is  relatively  uniform  and  persistent  over  long  distances 
both  in  strike  and  in  depth. 

ON  KEWEENAW  POINT,  MICHIGAN/  beds  of  copper  bearing 
conglomerate  occur  interstratified  with  sandstones  and  sheets  of 
diabase,  both  compact  and  amygdaloidal,  and  with  melaphyre; 
certain  beds  of  strongly  altered  diabase  scoriaceous  in  character 
are  known  as  ash  beds.  In  the  conglomerates,  the  copper  has 
replaced  the  finer  particles  so  as  to  appear  as  a  cement;  the 
boulders  themselves,  or  particular  minerals  in  them,  are  often 
permeated  with  copper,  which  occasionally  occurs  in  large 
masses.  The  copper  is  associated  with  chlorite,  epidote,  and 
abundant  zeolites.  The  associated  amygdaloidal  rocks  carry 
copper  in  their  small  cavities,  and  in  certain  shattered  areas  it 
occurs  irregularly,  occasionally  in  fragments  of  large  size.  The 
rich  parts  of  the  beds  occur  as  shoots,  in  some  instances  several 
thousands  of  feet  in  length;  these  shoots  continue  somewhat 
diagonally  down  the  dip  of  the  beds  to  great  depths  without 
essential  diminution  or  change  in  mineralization.  The  distri- 
bution of  the  copper  in  the  amygdaloidal  sheets  is  much  the 
same  as  in  the  conglomerate  beds. 

Bedded  Ore -Deposits. — Where  mineralization  has  proceeded 
contemporaneously  with  the  deposition  of  the  enclosing  bed  the 
resulting  deposit  is  known  as  a  seam,  or  bedded  deposit;  in  this 
class  are  also  included  those  replacements  of  similar  occurrence 
the  origin  of  whose  mineralization  is  not  apparent.  The  criteria 
for  distinguishing  between  bedded  deposits  and  intercalated  veins 

1  J.  P.  Kemp,  "Ore  Deposits/'  p.  204. 


TYPES  OF  PRIMARY  ORE-DEPOSITS  111 

are  given  in  a  preceding  paragraph.  •  Beck  makes  a  further 
distinction1  between  interbedded  deposits,  which  are  overlain 
by  other  strata,  and  superficial  deposits  where  there  are  no  over- 
lying beds,  as,  for  example,  beds  of  bog  iron  ore. 

Bedded  deposits  are  occasionally  recognizable  by  contained 
fossils,  which  may  have  become  mineralized.  These  deposits, 
of  which  a  majority  contain  iron  or  manganese,  are  commonly 
of  large  horizontal  dimensions  as  compared  with  thickness,  and 
where  the  strata  are  folded,  they  follow  all  the  sinuosities  of  the 
containing  bed;  in  occurrence  they  are  comparable  with  coal 
seams.  A  study  of  the  stratigraphy  of  the  bed  containing  the 
ore  and  of  the  associated  strata  is  imperative  in  the  investigation 
of  such  deposits. 

Certain  types  of  bedded  deposits  terminate  through  wedging 
out  around  their  peripheries,  others  toward  their  edges  become 
gradually  poorer  through  the  occurrence  of  barren  partings, 
which  increase  in  proportion  to  the  ore  until  the  mass  becomes 
unpayable.  Bedded  deposits  whose  mineralization  was  con- 
temporaneous with  the  deposition  of  the  containing  stratum  are 
commonly  persistent  over  large  areas. 

Bedded  deposits  whose  mineralization  is  later  than  the  de- 
position of  the  containing  bed  are  less  likely  to  be  persistent 
over  large  areas,  and  their  contained  mineralization  is  likely 
to  have  been  controlled  by  some  constituent  of  the  bed,  such  as 
carbonaceous  material,  and  to  fail  over  such  parts  of  the  bed 
as  did  not  contain  such  preciptants.  Where  two  or  more  bedded 
deposits  occur  in  the  same  series  they  are  persistently  parallel 
through  all  the  sinuosities  of  the  associated  strata. 

THE  CLINTON  ORE  MEASURES  OF  THE  UNITED  STATES/  are  geo- 
graphically persistent  in  extent,  and  wherever  they  outcrop  they 
almost  invariably  contain  one  or  more  beds  of  red  hematite 
intercalated  with  sandstones  and  shales.  In  occurrence  the  ore 
varies  somewhat,  at  times  being  the  replacement  of  fossils,  again 
as  small  oolitic  concretions,  and  in  places  constituting  a  highly 

1  Beck- Weed,  "Nature  of  Ore  Deposits,"  p.  49. 

2  J.  F.  Kemp,  "Ore  Deposits,"  p.  115. 


112 


EXAMINATION  OF  PROSPECTS 


ferruginous  limestone.  In  some  cases  it  can  be  shown  that 
these  beds  result  from  the  weathering  near  the  surface  of  beds 
of  ferruginous  limestone,  and  are  thus  secondary  or  residual  in 
character.  They  constitute  deposits  of  great  economic  import- 
ance in  several  states. 


Calcareous  Sandstone 

and 
thin  Shale  Layers    60'+ 


Non-Oolitic  Ore 
(Bed  Flux)  6' 


Calcareous 
Sandstone   6 ' 


Blue  Shale 

and  thin 
Sandstone  Layeis  15 ' 


Oolitic  Ore  2' 

Shale  2' 
Oolitic  Ore  l' 
Blue  Shale 
aud  thin 
Sandstone  Layers  100  -f- 


FIG.  51.  —  Section  of  the  Clinton  ore  measures,  Clinton,  New  York,  showing 
the  bedded  iron  ores.     After  C.  H.  Snvtth.    JT'  *       -  ' 


NEAR  MANSFELD;  GERMANY/  a  stratified  copper  deposit  extends 
over  an  area  120  miles  long  and  60  to  90  miles  wide;  the  average 
thickness  of  the  bed  varies  between  18  and  23  inches;  the  copper 
bearing  member  is  a  blackish,  bituminous  shale  which  lies  un- 
conformably  upon  red  sandstones  and  conglomerates,  and  is 
overlain  by  limestones  and  dolomites.  The  copper  occurs  in  the 

1  Beck-  Weed,  "Nature  of  Ore  Deposits,"  p.  488. 


TYPES  OF  PRIMARY  ORE-DEPOSITS 


113 


form  of  chalcopyrite,  bornite  and  chalcocite,  associated  with 
pyrite,  galena,  zincblende  and  other  sulphides,  and  contains  a 
little  silver.  Although  the  entire  bed  is  copper-bearing,  the 
payable  ore  is  commonly  confined  to  a  layer  from  3  to  6  inches 
in  thickness,  and  appears  to  be  associated  with  the  bituminous 
material,  the  bed  becoming  leaner  in  its  lower  portion  where 
the  carbonaceous  material  becomes  less  in  quantity;  replace- 
ments of  fossils,  especially  those  of  fishes,  are  frequent.  The 
average  value  of  the  ore  appears  to  be  from  2  to  3  per  cent, 
copper,  with  some  silver. 


Prospecting 


August 

Shaft 


FIG.  52. — Section  of   a  partly  eroded   anticline  near  Mansfeld,  Germany, 
showing  the  Mansfeld   copper  shale  as  a  black  line.     After  Schrader. 

THE  RED  SANDSTONE  BEDS  OF  THE  SOUTHWESTERN  UNITED 
STATES. — These  beds  at  many  places  carry  copper  as  chalcocite 
with  a  little  pyrite  and  chalcopyrite  replacing  organic  matter, 
such  as  tree  trunks,  leaves,  bark,  and  associated  finer  particles. 
Occasionally,  the  kaolin  or  calcite  of  the  cementing  material  be- 
tween the  grains  of  the  sandstone  is  replaced.  These  deposits 
appear  to  be  unconnected  with  fissures  and,  except  at  Tularosa, 
N.  M.,  are  not  in  association  with  intrusives.  The  chalcocite, 
which  commonly  carries  a  little  silver,  is  evidently  of  later  origin 
than  the  containing  beds,  but  the  source  of  mineralization  is  not 
apparent.  These  deposits,  while  frequently  confined  to  a  single 
bed,  are  commonly  present  in  several  beds  of  the  sandstone 
series.  The  mineralization  is  extremely  irregular,  and  these 
deposits  have  not  yielded  satisfactory  results  on  exploration. 


CHAPTER  VI 
PRIMARY  ORE -SHOOTS 

A  preceding  paragraph  treats  of  the  irregular  manner  in 
which  ore-deposits  occur  and  the  complex  factors  that  control 
their  distribution;  the  occurrence  of  metals  in  ore-shoots  in 
individual  deposits  is  equally  irregular,  and  the  factors  that 
control  such  segregation  are  equally  complex.  While  the  dimen- 
sions of  ore-shoots  in  individual  deposits  may  not  be  foretold 
with  any  degree  of  accuracy,  the  occurrence  of  ore  in  shoots  is 
not  accidental,  but  is  controlled  by  laws  some  of  which  are 
understood.  Any  one  familiar  with  permutations  and  combina- 
tions who  stops  to  consider  the  variety  of  factors  that  control 
ore  deposition,  and  their  varying  relative  importance,  will  admit 
that  the  science  of  economic  geology  must  advance  greatly 
before  the  occurrence  of  ore-shoots  may  be  predicted  accurately 
in  advance  of  exploration. 

The  Factors  that  Determine  Primary  Ore -Shoots. — The  segrega- 
tion of  minerals  in  any  ore-shoot  must  be  referred  to  some  con- 
dition, or  combination  of  conditions,  local  to  the  ore-shoot  as 
compared  with  the  remainder  of  the  deposit  of  low-grade  or 
barren  minerals.  Among  the  localizing  factors  that  have  been 
recognized  are,  in  filled  fissures,  the  amount  and  distribution  of 
open  spaces  available  for  ore  deposition,  and  in  replacement 
deposits,  the  degree  of  brecciation  giving  access  to  the  replacing 
solutions;  the  intersection  of  veins  with  other  veins,  dikes, 
sheeted  zones/or  porous  strata;  the  impounding  of  solutions  by 
impervious  strata;  and  the  differing  precipitative  influences  of 
enclosing  rocks.  A  large  proportion  of  important  and  well- 
defined  ore-shoots,  especially  in  deposits  of  deep-seated  origin, 
.  are  not  assignable  to  any  of  these  factors. 

The  Relative  Value  of  General  and  Local  Data. — While  a 

114 


PRIMARY  ORE-SHOOTS  115 

knowledge  of  the  character  of  ore-shoots  in  general  is  important, 
a  knowledge  of  the  behavior  of  the  known  ore-shoots  in  the  prop- 
erty or  in  the  district  under  investigation  is  even  more  signifi- 
cant. Accurate  and  complete  assay  and  geological  maps  of  the 
areas  already  mined  are  essential  to  an  intelligent  investigation 
of  ore-shoots,  and  it  is  strange  that  such  maps  are  so  rarely  to  be 
found  at  even  important  mines. 

The  distribution  of  ore  in  shoots  is  the  greatest  factor  in  the 
risk  of  mining,  and  the  factor  of  most  vital  importance  to  the 
mine  owner  and  mining  engineer.  Not  infrequently,  a  practical 
man  from  long  experience  in  a  particular  deposit  can  predict  its 
behavior  in  advance  of  exploration  with  a  valuable  percentage  of 
accuracy;  this  reliability  of  prediction  would  in  many  cases  be 
greatly  increased  by  a  technical  study  of  complete  assay  and 
geological  maps,  upon  which  relations  become  definite  that  other- 
wise would  not  suggest  themselves. 

Ore-shoots  vary  with  commercial  as  well  as  with  natural 
conditions,  and  material  left  in  place  as  waste  may  in  later  years 
constitute  a  valuable  ore.  Furthermore,  while  the  difference 
between  assays  of  20  cents  and  $1.00  is  not  of  immediate 
commercial  importance  as  regards  the  marketability  of  an  ore, 
it  may  be  of  vital  importance  as  a  guide  to  a  new  ore-shoot.  It 
is  natural  to  extract  ore  and  to  be  satisfied  as  long  as  the  .ore  lasts, 
but  all  ore-shoots  come  to  an  end,  and  then,  when  a  search  is 
commenced  for  further  reserves,  the  data  of  most  value  in  con- 
ducting that  search  will  have  been  lost  unless  carefully  recorded 
on  a  map,  and  the  exploration  must  be  conducted  by  a  process 
of  elimination  which  may  or  may  not  yield  results. 

Primary  and  Secondary  Ore -Shoots. — In  the  study  of  any 
deposit  the  first  question  to  decide  is  whether  the  ore-shoots  are 
due  to  primary  segregation  of  values  or  to  secondary  enrichment 
by  surface  agencies.  . 

It  is  much  easier  to  recognize  a  secondary  ore  with  certainty 
than  to  determine  an  ore  to  be  certainly  primary,  for  the  reason 
that  the  criterion  of  a  primary  ore  is  the  known  lack  of  secondary 
additions.  With  a  majority  of  ores  microscopic  study  in  thin 


116 


EXAMINATION  OF  PROSPECTS 


section  will  reveal  primary  character,  but  there  are  many  instances 
where  the  assumed  primary  nature  of  an  ore  is  open  to  question, 
especially  with  ores  of  gold  or  silver. 

Even  below  the  zone  of  recognized  secondary  enrichment,  deep 
exploration  usually  shows  a  falling  off  in  value  in  the  primary 
ore  in  depth,  and  the  actual  influence  of  surface  agencies  may 
extend  to  greater  depths  than  is  generally  supposed.  It  often  hap- 
pens that  commercially  valuable  ore  ceases  at  the  depth  reached 
by  secondary  agencies,  and  the  data  is  likely  to  be  scanty  in 


Cross  Section 


Longitudinal  Section 


Level  1 


MR  Width  or 
thickness 


I  Level  3 


Level  I 


Level  2 


Level  3 


FIG.  53. — Diagram  illustrating  the  terms  used  to  describe  the  dimensions  of 
ore-shoots.     After  Lindgren  and  Ransome. 

regard  to  the  primary  distribution  of  values  where  not  obscured 
by  surface  agencies.  Secondary  ore-shoots  will  be  considered  in 
a  succeeding  chapter. 

Terms  Used  to  Describe  the  Dimensions  of  Ore -Shoots. — The 
most  convenient  terms  for  use  in  describing  ore-shoots  are  those 
suggested  by  Messrs.  Lindgren  and  Ransom.1  The  longest 
dimension  of  a  shoot  is  called  the  "pitch  length";  the  horizontal 
length  the  "stope  length";  the  width  at  right  angles  to  the 

'P.  P.  54,  U.  S.  G.  S.,   p.  205. 


PRIMARY  ORE-SHOOTS 


117 


118 


EXAMINATION  OF  PROSPECTS 


pitch  length  is  termed  the  "breadth";  thickness  is  measured  at 
right  angles  to  the  plane  of  the  pitch  length  and  breadth. 

The  Shapes  of  Ore -Shoots. — Probably  no  mineral  lode  ap- 
proaches the  regularity  of  a  seam  of  coal,  although  as  compared 
with  the  maj  ority  of  mineral  veins  some  are  notably  regular.  The 
Smuggler  vein  at  Telluride,  Colorado,  and  the  older,  East- West,1 


FIG.  55. — Primary  ore-shoots,   Grass  Valley,   California.     After  Lindgren. 

veins  at  Butte,  Montana,  are  remarkably  regular  and  uniform 
in  mineralogical  character;  the  more  prominent  of  these 
veins  have  been  stoped  for  thousands  of  feet  along  their  strike, 
showing  little,  if  any,  disposition  to  develop  shoots;  the  gold 
reefs  of  the  Rand,  South  Africa,  are  markedly  regular  over  long 
distances  as  compared  with  most  lodes.  These  examples,  while 
of  marked  general  regularity,  show  considerable  variation  in 
value  at  different  points.  The  other  extreme,  that  of  irregular- 
ity, is  more  common,  and  instances  are  numerous  where  the 

1  R.  H.  Sales,  Economic  Geology,  Vol.  Ill,  p.  327. 


PRIMARY  ORE-SHOOTS  119 

valuable  metals  occur  in  small  high-grade  masses  so  irregularly 
distributed  through  the  gangue  that  they  may  not  be  developed 
ahead  of  extraction,  but  must  be  discovered  by  chance  and 
mined  out  as  found.  Between  these  two  extremes  may  be  classed 
a  majority  of  ore-deposits,  where  the  payable  ore  occurs  in  fairly 
definite  shoots,  the  size  and  shape  of  which  depend  upon  the 
market  prices  of  metals  and  the  cost  of  mining  as  well  as  on 
geological  conditions. 

The  broadest  generalization  that  can  be  made  in  regard  to  the 
shape  of  primary  ore-shoots  is  that  their  vertical  dimension  is 
likely  to  exceed,  even  considerably  exceed,,  their  horizontal 
dimensions:  in  secondary  ore-shoots  the  reverse  is  the  rule. 
There  are  many  instances  where  primary  ore-shoots  are  as  well 
defined  in  vertical  as  in  horizontal  extent,  as,  for  example,  are 
many  of  ore-shoots  at  Cripple  Creek,  Colorado;1  here  the  ratio 
between  pitch  length  and  breadth  varies  from  1  1/2  to  1  to  5  to  1 
in  the  shoots  that  have  not  been  truncated  by  erosion. 

Some  generalizations  have  been  attempted  in  regard  to  the 
relative  size  of  ore-shoots  at  the  surface  and  in  depth:  this  applies 
with  some  force  to  secondary  ore-shoots  and  to  those  primary  ore 
shoots  whose  upper  parts  have  been  leached.  It  is  evident, 
however,  that  most  primary  ore-shoots  were  formed  at  very 
considerable  depths  and  that  the  present  surface  must  be  con- 
sidered accidental  and  therefore  not  a  factor  in  determining  the 
primary  distribution  of  metals.  Where  primary  ore-shoots  are 
definitely  limited  in  vertical  extent  they  are  likely  to  be  of 
roughly  lenticular  form,  and  other  shoots  are  likely  to  appear 
in  the  projection  of  their  piteh  lengths  in  depth.  In  some 
districts  primary  ore-shoots  are  persistent  to  great  depths, 
although  of  well-defined  breadth. 

No  valuable  generalizations  can  be  formulated,  but  there  is 
often  an  approach  to  regularity  in  the  relationship  among  the 
ore-shoots  of  individual  mines  or  districts,  as  regards  pitch, 
continuity,  relative  pitch-length  to  breadth,  and  in  the  regularity 
of  sequence  of  different  shoots.  The  effect  of  structural  con- 

1  Waldemar  Lindgren  and  F.  L.  Ransome,  P.  P.  54  U.  S.  G.  S.,  p.  205. 


120 


EXAMINATION  OF  PROSPECTS 


ditions  upon  ore-shoots  and  the  chemical  effect  of  the  wall  rocks 
in  inducing  precipitation  will  be  taken  up  in  a  succeeding  para- 
graph. Where  these  factors  do  not  appear  to  have  had  effect 
in  the  distribution  of  metals,  the  causes  of  their  segregation 
into  shoots  are  not  understood,  unless  they  are  referred  to  mass 
action,  where  a  precipitation  once  started,  by  whatever  cause, 
becomes  a  continuous  process  until  the  final  result  is  an  ore-shoot. 


West 


East 


South 


North 


Q      100     200 


•400  feet 


FIG.  56. — Sections  of  the  Silver  Bell  ore-shoot,  Silverton,  Colorado.     After 

Ransome. 

A  certain  type  of  ore-shoot  gradually  fades  out  around  its 
peripheries,  the  values  becoming  progressively  less  from  the 
center  toward  the  edges,  where  the  final  boundary  of  the  shoot 
is  determined  by  the  limiting  cost  of  mining  and  treatment. 
This  fading  out  may  be  accompanied  by  a  visible  change  in  the 
mineralogy  of  the  shoot,  or  the  same  gangue  and  acessory  minerals 
may  persist  beyond  the  payable  values;  the  vein  may  pinch 


PRIMARY  ORE-SHOOTS  121 

out  with  the  diminishing  values,  or  may  continue  as  strongly 
beyond  them.  Another  type  of  shoot  presents  sharp  outlines, 
the  grade  of  ore  being  maintained  up  to  the  boundaries  of  the 
shoot. 

In  many  wide,  compound  lodes  or  shear  zones  the  ore-bodies 
occur  with  great  individual  irregularity,  either  connected  with 
each  other  by  stringers  or  apparently  unconnected,  and  lying 
either  along  the  foot-  or  the  hanging-wall;  taken  collectively, 
however,  they  are  likely  to  show  a  general  alignment,  or  arrange- 
ment in  shoots,  and  the  lode  in  the  direction  of  their  collective 
pitch  lengths  may  be  considered  promising  territory. 

Ore-shoots  not  infrequently  follow  an  overlapping  arrange- 
ment where  each  shoot  overlaps  the  one  succeeding  on  one  side 
and  the  one  following  on  the  other.  Where  there  are  several 
parallel  fissures  an  ore-shoot  on  one  vein  is  likely  to  be  succeeded 
by  a  slightly  overlapping  shoot  in  the  adjoining  vein. 

Lenticular  Ore -Shoots. — Lenticular  masses  of  ore  in  schistose 
rocks  constitute  another  class  of  primary  ore-shoots;  here  the 
original  ore-body,  of  whatever  origin,  has  been  transformed  by 
regional  metamorphism  into  lenticular  masses  oriented  with 
their  plane  of  greatest  dimension  roughly  parallel  to  the  schistos- 
ity.  These  lenticular  masses  are  likely  to  follow  one  another  in 
overlapping  sequence,  but  often  occur  singly. 

The  lenticular  form  is  often  well-defined,  the  central  part 
being  thickest,  and  the  ore  gradually  diminishing  in  thickness 
toward  the  edges,  where  the  lens  dies  out  in  the  shist. 

During  the  formation  of  these  lenses  recrystallization  appears 
to  destroy  the  lines  of  schistosity  through  the  mass  of  the  ore, 
but  the  schistose  structure  is  commonly  present  near  the  bound- 
aries, where  the  ore  is  often  mingled  with  parallel  bands  of  the 
minerals  of  the  enclosing  schist.  In  pyritic  lenses  carrying 
copper,  a  part  of  the  chalcopyrite  is  likely  to  be  segregated  and 
to  occupy  veinlets  through  the  pyrite. 

In  many  cases  lenticular  ore-bodies  appear  to  have  controlled 
the  schistosity  of  the  enclosing  rock,  and  in  the  exploration  for 
further  lenses  a  plotting  of  the  schistosity  is  often  helpful.  Mr. 


122 


EXAMINATION  OF  PROSPECTS 


W.  H.  Emmons1  states  that  where  such  deposits  are  separated 
to  form  overlapping  lenses,  there  is  some  evidence  to  show  that 
they  separate  where  the  ore  is  most  siliceous,  for  a  mixture  of 
sulphides  and  quartxz  seems  to  be  less  capable  of  resisting  stresses 
than  the  more  massive  pyrite.  In  such  cases  a  thin  fissure 
with  the  schistosity  parallel  on  either  side  will  probably  be  found 
to  connect  the  broken  ends. 


FIG.  57. — Section  showing  the  lenticular  masses  that  make  up  the  Conti- 
nental vein,  Encampment,  Wyoming.    A/ter  Spencer. 

The  Behavior  of  Primary  Ore -Shoots  in  Depth. — A  majority 
of  the  ore-deposits  of  our  Western  States  are  believed  to  be  the 
result  of  ascending  mineral-bearing  solutions  which  deposited 
their  burdens  where  diminishing  temperature  and  pressure 
permitted  precipitation  upon  nearing  the  surface. 

Assuming  a  uniform  fissure  through  a  homogeneous  rock,  it 
would  seem,  therefore,  that  the  changes  to  be  expected  in  depth 
would  be  those  due  to  changes  in  pressure  and  temperature,  the 
most  easily  precipitated  mineral,  or  the  mineral  in  greatest 
excess,  extending  the  deepest,  with  the  other  minerals  following 
in  regular  sequence  until  near  the  surface  the  most  soluble 
mineral  would  be  found.  This  is  partly  borne  out  by  recorded 
facts,  but  no  absolute  succession  of  minerals  maybe  distinguished, 

1  Economic  Geology,  Vol.  VI,  p.  781. 


PRIMARY  ORE-SHOOTS  123 

for  the  reason,  possibly,  that  the  solubilities  of  the  different 
elements  and  compounds  vary  greatly  according  to  the  presence 
of  other  compounds  in  solution,  and  also  by  reason  of  the 
interference  of  structural  conditions  and  the  chemical  effects  of 
enclosing  wall  rocks.  The  usual  order  is,  nearest  the  surface, 
compounds  of  mercury,  then  galena,  blende,  chalcopyrite  or 
cupriferous  pyrite,  with  pyrite  the  lowest.  These  zones  all 
grade  one  into  the  other,  and  in  many  cases  but  little  change 
is  noted  in  the  relative  abundance  of  these  minerals  over  con- 
siderable vertical  distances.  Gold  appears  to  be  precipitated 
at  all  depths,  free  gold  and  auriferous  pyrite  extending  the  deepest, 
and  the  compounds  of  gold  of  greatest  mobility,  such  as  tellu- 
rides,  extending  the  highest.  The  position  of  silver  appears 
to  be  higher  up  in  the  series.  In  the  absence  of  arsenic,  anti- 
mony, bismuth  and  tellurium,  silver  is  usually  associated  with 
galena,  but  in  the  presence  of  these  elements,  it  is  likely  to  be 
higher  up  in  the  series  and  combined  with  these  elements  in 
preference  to  lead.  The  primary  character  of  certain  of  the 
silver  minerals  is  open  to  question.  This  sequence  is  partially 
corroborated  by  the  typical  associations  of  the  metals.  Of 
the  six  elements,  silver  is  most  commonly  associated  with  galena, 
galena  with  zincblende,  zincblende  with  chalcopyrite  (to  a  less 
marked  degree),  chalcopyrite  with  pyrite,  and  pyrite  with  gold. 

The  general  tendency  is  for  ore-shoots  to  become  smaller,  but 
more  regular,  with  increase  in  depth. 

The  Decrease  in  Value  with  Depth. — Primary  ores  commonly 
show  a  progressive  decrease  in  value  in  depth.  Mr.  Waldemar 
Lindgren  says1  "This  decrease  is  likely  to  be  rapid  near  the 
original  apex  of  the  veins,  but  below  this  it  is  in  most  cases  very 
slow,  extending  over  a  vertical  range  of  many  thousand  feet." 
As  examples,  he  gives  the  Grass  Valley  Mines,  California, 
where  the  North  Star  vein  produces  a  gold  ore  from  a  vertical 
depth  of  1600  ft.,  which  corresponds  to  4100  ft.,  on  the  incline, 
equally  as  rich  as  that  found  at  higher  levels.  The  saddle  reefs 

J  Economic  Geology,  Vol.  I,  p.  45. 


124  EXAMINATION  OF  PROSPECTS 

at  Bendigo,  Australia,  contain  payable  ore  at  a  depth  of  4156  ft. 
In  the  Coeur  D'Alenes,  Idaho/  the  lead-silver  ores  appear  to 
maintain  their  tenor  to  depths  of  1800  ft.  below  their  outcrops 
without  sign  of  diminishing  values. 

If  any  generalization  may  be  made,  it  would  seem  that  silver, 
lead  and  zinc  are  less  likely  than  gold  to  be  persistent  over  great 
vertical  distances. 

Predicting  the  Depth  to  which  Ore -Shoots  may  Continue. — The 
primary  character  of  an  ore  being  established,  and  the  effect  of 
structural  and  chemical  precipitants  not  being  apparent,  the 
chances  are  good  that  an  ore-shoot  will  maintain  its  values  in 
depth.  A  shoot  whose  partial  development  indicates  a  widen- 
ing tendency,  or  fairly  uniform  breadth  through  several  levels, 
has,  of  course,  a  much  better  chance  of  vertical  persistency  than 
one  that  is  definitely  narrowing  as  it  goes  down,  which  probably 
belongs  to  the  type  of  ore-shoot  that  has  definite  vertical  as 
well  as  horizontal  limits.  If  the  variation  in  value  along  the 
dip  is  no  more  pronounced  than  along  the  strike,  there  is  no 
basis  upon  which  to  presuppose  the  absence  of  ore  in  the  deeper 
portions  of  the  vein,  where  further  exploration  may  disclose 
other  ore-shoots,  most  likely  in  the  projection  of  the  pitch  length 
of  a  shoot  higher  up  on  the  vein.  It  has  been  established,  how- 
ever, that  in  many  instances  where  the  values  are  maintained 
in  depth,  the  absolute  quantity  of  ore  grows  less  with  increasing 
depth,  which  may  be  a  condition  more  apparent  than  real,  on 
account  of  the  greatly  increased  expense  of  deep  exploration. 

The  Depths  at  which  Ore -Deposits  Form. — Ore-deposits  have 
been  classified  on  the  basis  of  the  depths,  or  the  conditions, 
under  which  they  were  formed.  Deposits  of  igneous,  pegmatitic 
or  of  contact  origin,  and  of  abyssal,  moderate  and  of  shallow 
depths  exhibit  characteristic  mineral  associations,  which  fre- 
quently permit  the  investigator  to  assign  an  ore-deposit  to  one 
of  these  classes. 

A  knowledge  of  the  depth  at  which  an  ore  was  formed  gives  an 

1  F.  L.  Ransome,  P.  P.  62,  U.  S.  G.  S.,  p.  130. 


PRIMARY  ORE-SHOOTS  125 

idea  of  the  amount  of  erosion  that  took  place  to  expose  the 
deposit  at  the  surface,  and  permits  an  intelligent  corelation  to  be 
made  with  deposits  of  similar  origin  elsewhere. 

The  Structural  Features  that  Influence  Ore -Shoots. — With  the 
exception  of  magmatic  segregations  and  deposits  due  to  contact 
metamorphism,  a  prerequisite  of  mineralization  is  the  existence 
of  a  fissure  or  fissures  to  give  access  to  the  mineralizing  solutions; 
the  mineralizing  effect  of  these  solutions  is  probably  controlled 
to  a  large  degree  by  the  character  and  changes  of  these  circu- 
lation channels. 

One  extreme  of  fracturing  may  be  considered  the  solid,  un- 
broken rock,  through  which  solutions  work  their  way  very 
slowly.  The  other  extreme  is  a  crushed  or  ground-up  condition 
where  the  fineness  of  comminution  is  sufficient  to  produce  a 
gouge  or  clay-like  mass,  which  is  also  relatively  impervious  to 
the  passage  of  solutions.  Between  these  two  extremes  lie  the 
favorable  conditions  for  ore  deposition. 

Ore  is  deposited  from  solutions  either  in  open  spaces  or  by 
replacement  of  the  fissured  rock.  In  replacement  deposits 
the  degree  of  mineralization  is  frequently  proportional  to  the 
degree  of  brecciation,  as  replacement  proceeds  most  rapidly 
where  it  has  the  greatest  area  of  rock  surfaces  on  which  to  work, 
until  the  point  is  reached  where  the  fineness  of  comminution 
commences  to  retard  the  passage  of  the  solutions.  There  is, 
however,  a  point  where  brecciation  ceases  to  be  an  advantage — 
where  it  is  so  extensive  that  the  solutions  are  dissipated  through 
a  large  mass,  and  the  resulting  mineralization  is  too  scattered 
to  yield  a  commercially  important  deposit. 

It  sometimes  happens  that  two  or  more  systems  of  fracturing 
or  brecciation  have  effected  the  rocks;  here  the  time  of  the  fissur- 
ing  becomes  important.  Fissures  heal  and  become  closed  in 
time,  and  so  cease  to  afford  circulation  channels;  the  most 
favorable  time  of  fissuring  or  brecciation  may  be  considered 
that  just  before  the  introduction  of  the  solutions — perhaps 
due  to  the  same  igneous  disturbances  to  which  the  solutions 
owe  their  origin.  Post-mineral  fracturing  is,  of  course,  of  no 


126  EXAMINATION  OF  PROSPECTS 

primary  effect,  but  may  be  of  the  greatest  importance  in  working 
secondary  changes. 

In  the  investigation  of  a  mineralized  area,  therefore,  the  dis- 
tribution and  extent  of  the  brecciated  masses,  and  the  degree 
and  relative  time  of  brecciation  should  be  studied;  mineralization 
is  frequently  co-extensive  with  or  confined  to  the  brecciated 
areas.  The  brecciated  structure  of  an  unmineralized  rock  is 
sometimes  not  visible  on  fresh  fractures,  but  becomes  plain  on 
weathering,  or  upon  being  wetted. 

The  degree  of  brecciation  varies  greatly  in  different  rocks; 
massive  or  rigid  rocks  yield  breccias  where  soft  or  plastic  rocks 
yield  to  strain  without  brecciation.  The  continuity  of  a  brec- 
ciated zone  from  one  rock  into  another  is,  therefore,  uncertain. 

The  degree  and  character  of  brecciation  is  of  the  greatest 
importance  in  the  study  of  the  surface  exposures  where  the 
existence  of  disseminated  copper  ores  is  suspected. 

AT  BINGHAM,  UTAH,  Mr.  J.  M.  Boutwell,1  referring  to  the  dis- 
seminated mineralization  of  the  great  laccolithic  mass,  states 
that:  "In  general,  underground  observations  tend  to  show  that 
chalcopyrite  and  pyrite  occur  in  greatest  quantity  where  the 
country  rock  has  been  most  broken." 

AT  ASPEN,  COLORADO,  referring  to  the  silver  lodes,  Mr.  J.  E. 
Spurr2  states  that:  "A  microscopic  study  of  limestone  in  the 
process  of  replacement  by  dolomite,  silica,  and  sulphides  shows 
that  the  rock  was  first  profoundly  strained  and  crushed  in  the 
vicinity  of  faults,  so  that  many  tiny  passages  were  opened  to 
solutions,  which  finally  worked  through  and  through  the  strained 
material,  replacing  it.  The  amount  of  straining,  which  regu- 
lated the  area  of  surface  offered  to  the  solutions,  usually  deter- 
mined whether  there  would  be  little  or  much  replacement; 
where  the  circulation  zone  along  a  vein  is  wide  and  is  filled 
largely  with  finely  ground  material  which  is  not  so  pasty  as  to 
prohibit  free  circulation  through  it,  the  replacement  of  earthy 
materials  by  metallic  minerals  as  well  as  their  accompanying 


1  P.  P.  38,  U.  S.  G.  S.,  p. 

2  P.  P.  63,  U.  S.  G.  S.,  p. 


131. 
160. 


PRIMARY  ORE-SHOOTS  127 

gangue  goes  on  very  much  more  rapidly  than  elsewhere  along 
the  vein,  where  the  rock  is  hard  and  is  only  sliced  by  fracture 
planes." 

Ore -Shoots  due  to  Available  Open  Space. — In  deposits  formed 
by  the  filling  of  open  spaces  the  distribution  of  these  open  spaces 
is  usually  the  controlling  factor  in  ore  deposition,  and  the  segre- 
gation of  metals  into  ore-shoots  bears  the  closest  relation  to  the 
vein  structure. 

The  pinching  and  swelling  of  veins  in  fissures  of  small  dis- 
placement have  been  discussed  in  a  preceding  chapter;  where 
this  condition  can  be  shown  to  exist,  the  pinching  out  of  an  ore- 
shoot  may  be  reasonably  expected  to  be  followed  by  at  least  a 
widening  of  the  vein,  which  may  or  may  not  contain  ore. 

A  vein  that  is  the  result  of  both  replacement  and  of  the  filling  of 
open  spaces  is  likely  to  vary  in  mineral  content  according  to 
which  of  the  two  processes  took  place  during  ore  deposition, 
the  open  spaces  perhaps  contributing  the  ore-shoots  while  the 
replacement  of  the  walls  where  the  fissure  was  tight  is  represented 
by  intervals  of  barren  or  low-grade  filling. 

In  a  crustified  vein,  if  it  is  determined  that  the  valuable 
mineral  was  among  the  first  formed,  and  relatively  near  the 
walls,  the  ore  is  likely  to  be  persistent  as  compared  with  a 
mineral  that  was  among  the  last  to  form  and  so  confined  to 
such  spaces  as  were  kept  open  the  longest. 

Ore -Shoots  due  to  Intersections. — Ore-shoots  are  frequently 
developed  where  a  vein  is  intersected  by  another  vein  or  by  a 
fissure,  dike,  sheeted  zone,  or  branch  vein.  This  association  is 
more  common  in  deposits  that  have  been  formed  near  the 
surface  than  in  the  deposits  of  deep  seated  origin.1 

Intersecting  members  appear  to  effect  the  conditions  of  circu- 
lation, or  to  introduce  chemical  or  physical  changes  that  acceler- 
ate the  deposition  of  ore  minerals  along  such  junctions. 

An  intersection  may  afford,  through  increased  local  crushing, 
a  more  open  channel  for  the  passage  of  solutions,  and  so  lead  to 

1  See  Economic  Geology,  Vol.  I,  p.  43,  Waldemar  Lindgren. 


128 


EXAMINATION  OF  PROSPECTS 


FIG.  58. — Longitudinal  section  along  the  Neu-Hoffnung  vein,  Him- 
melsfahrt,  Germany,  showing  the  ore-shoots  along  the  intersections  with 
several  other  veins.  A/ter  Beck. 


PRIMARY  ORE-SHOOTS  129 

the  passage  of  a  greater  quantity  of  the  mineralizing  solutions 
through  the  part  of  the  vein  effected  by  the  intersection; 
moreover,  a  partial  stoppage  of  circulation  appears  to  induce 
precipitation,  stagnation,  or  partial  stagnation,  being  favorable 
to  precipitation.  In  some  instances  the  intersecting  veins 
carried  solutions  of  different  metal  content,  as  is  evidenced  by 
their  different  vein  fillings,  and  through  a  mingling  of  the  two 
solutions  chemical  precipitation  was  brought  about  that  formed 
ore-shoots:  furthermore,  a  mingling  of  solutions  under  different 
conditions  of  temperature  or  pressure,  appears  likely  to  disturb 
the  equilibrium  of  the  solution,  and  so  to  cause  new  or  increased 
local  precipitation.  In  some  cases  the  intersection  is  with  a 
mineralized  vein,  and  both  veins  along  their  intersection  and  for 
some  distance  back  from  it  carry  ore-shoots;  in  other  cases,  the 
intersection  is  with  a  barren,  or  unmineralized  fracture,  which 
appears  equally  likely  to  cause  a  segregation  of  values. 

The  relation  between  intersections  and  secondary  ore-bodies 
is  of  perhaps  even  greater  importance  than  in  primary  deposits. 

Dr.  Richard  Beck1  states  that  the  localization  of  values  is 
most  marked  where  veins  intersect  at  acute  angles,  because  the 
intersecting  vein  walls  are  close  together  for  longer  distances 
and  also  because  the  mass  of  crushed  and  permeable  rock  is 
likely  to  be  greater  in  acute  than  in  more  nearly  rectangular 
intersections. 

IN  THE  TORNADO-MOGUL  SlLICEOUS  ORE-SHOOT,  BLACK  HlLLS, 

SOUTH  DAKOTA/  the  large  north-south  ore-body  is  joined  on  the 
east  by  northeast  ore-shoots;  at  the  junctions  the  ores  carry 
more  gold  and  silver  than  do  any  of  the  shoots  away  from  the 
intersections. 

AT  CRIPPLE  CREEK,  COLORADO,S  the  smaller  ore-shoots  are  quite 
generally  associated  with  intersections;  the  large  ore-shoots 
appear  to  be  independent  of  intersections. 

1  Beck- Weed,  "Nature  of  Ore  Deposits/'  p.  391. 

2  J.  D.  Irving,  Economic  Geology,  Vol.  Ill,  p.  153. 

3Waldemar    Lindgren    and    F.    L.    Ransome,   P.  P.   54,   U.   S.   G.  S., 
p.   210-212. 
9 


130 


EXAMINATION  OF  PROSPECTS 


IN  THE  GEORGETOWN  QUADRANGLE,  COLORADO/  a  large  pro- 
portion of  the  important  ore-shoots  occur  at  intersections  with 
branch  veins. 

AT    TONOPAH,    NEVADA/    pre-mineral    cross    fractures    and 


Quartz-monzonite 
porphyry 


Black  carbon- 
aceous shale 
40  '± 


Clay  and 
sandy 


FIG.  59. — Section  of  the  rocks  and  diagram  of  the  ore-bodies  in  the 
American  Nettie  mine,  Ouray,  Colorado,  showing  the  localization  of  ore 
below  an  impervious  stratum  due  to  the  impounding  of  solutions.  After 
Irving. 

cross  walls  appear  to  have  been  the  cause  of  localizing  values 
in  ore-shoots,  which,  in  general,  pitch  in  a  direction  parallel 
to  the  intersections  of  these  minor  fissures  with  the  main  vein 
system.  To  quote  Mr.  Spurr:  "To  these  cross  walls,  more 

1  J.  E.  Spurr,  P.  P.  63,  U.  S.  G.  S.,  p.  159. 

2  J.  E.  Spurr,  P.  P.  42,  U.  S.  G.  S.,  p.  85,  and  119. 


PRIMARY  ORE-SHOOTS 


131 


or  less  pronounced,  was  due  the  division  of  the  water  circu- 
lation into  columns  of  unequal  importance,  and  the  minerali- 
zation accomplished  by  these  waters  was  correspondingly 
localized." 

Ore -Shoots  Due  to  the  Impounding  of  Solutions. — A  numerically 


Sandstone 
Sandy  shale 
Sandsrone 

Black  shale 
Blanket 

Blanket  limestone 
Black  shale 
Sandstone 
Sandy  shale 

Sandstone 

Sandy  shale 

Sandstone 
Sandy  shale 

Sandsrone 
Sandy  shale 

Sandstone 
Sandy  shale 


Lode 


FIG.  60. — Diagrammatic  section  across  a  lode  and  ore-body  formed  beneath 
an  impervious  stratum,  Rico,  Colorado.     After  Ransome. 

important  type  of  ore-body  appears  to  be  the  result  of  the  im- 
pounding of  mineralizing  solutions  beneath  impervious  cappings. 
Fissures  are  formed  only  in  rocks  that  are  relatively  rigid,  and 
are  likely  to  die  out  upon  encountering  a  stratum  or  mass  of  rock 
that  yields  to  the  forces  that  elsewhere  produce  fissures.  Most 
rocks  are  sufficiently  rigid  to  permit  the  development  of  fractures, 


132  EXAMINATION  OF  PROSPECTS 

and  the  only  rock  of  wide  distribution  that  yields  in  preference 
to  fracturing  is  shale;  as  shales  are  frequently  intercalated  with 
limestones  and  quartzites,  ore-bodies  due  to  the  impounding  of 
solutions  are  usually  replacments  of  these  rocks  and  the  develop- 
ment of  the  ore-shoots  may  be  considered  as  the  effect  of  both 
structural  and  chemical  causes. 

It  appears  that  where  a  mineralizing  solution  is  impounded, 
or  its  circulation  arrested,  the  increased  time  during  which  the 
mineralizing  agents  are  in  contact  with  the  rock  is  an  important 
factor  in  localizing  deposition.  The  fact  that  the  farther  upward 
passage  of  these  solutions  or  mineralizing  agents  was  almost, 
if  not  quite  checked,  indicates  that  the  flow  of  solutions  from 
below  could  not  have  continued  long;  large  ore-bodies  formed 
beneath  impervious  strata  would  seem  to  indicate,  therefore,  that 
the  mineralizing  agents  possessed  greater  mobility  than  liquid 
solutions,  and  perhaps  rose  through  such  stagnant  solutions  to 
perform  the  work  of  ore  deposition. 

IN  THE  OURAY  DISTRICT,  COLORADO/  fissures  of  small  displace- 
ment that  are  well  developed  in  limestone  and  quartzite  strata 
become  lost  upon  meeting  black  shales,  where  they  are  repre- 
sented by  slight  distortions  of  the  beds  only.  Beneath  these 
impervious  shale  beds  important  replacement  deposits  have 
formed  in  the  limestones  and  quartzites,  apparently  due  to  the 
impounding  of  rising  solutions. 

IN  THE  BLACK  HILLS,  SOUTH  DAKOTA/  the  vertical  fissures  pass 
through  limestone  into  impervious  shales:  the  ore-bodies,  which 
are  replacements  of  the  limestone,  are  widest  just  beneath  the 
shales,  and  narrowest  in  their  downward  extension,  apparently 
from  the  impounding  of  rising  waters.  In  the  great  Tornado- 
Mogul  ore-shoot  this  occurs  on  a  large  scale.  Here  a  phonolite 
dike  forms  the  west  wall  of  the  ore-body,  and  not  only  has  the 
upward  progress  of  the  solutions  been  prevented  by  overlying 
shales,  but  they  have  been  confined  laterally  also  by  the  por- 
phyry wall. 

1  J.  D.  Irving,  Bull  260,  U.  S.  G.  S.,  pp.  58,  62. 

2  J.  D.  Irving,  Economic  Geology,  Vol.  Ill,  p.  150,  and  P.  P.  26,  U.  S.  G.  S. 


PRIMARY  ORE-SHOOTS 


133 


IN  GRANT  COUNTY,  NEW  MEXICO/  silver,  and  silver-gold  ores 
occur  in  limestone  immediately  below  strata  of  shale,  which,  by 
impounding  solutions,  have  had  a  distinct  effect  in  the  localization 


FIG.  61. — Sketch  showing  narrow  mineralizing  fissures  and  the  replace- 
ment of  limestone  by  ore  beneath  a  stratum  of  impervious  shale,  Alameda 
mine,  Portland,  South  Dakota.  After  Irving. 


FIG.  62. — Section  along  the  West  Side  Vein,  Tombstone,  Arizona,  showing 
the  ore-bodies  developed  at  the  tops  of  anticlines.     After  Church. 

of  the  ore-bodies  at  Kingston,  Lake  Valley,  Hermosa,  and  other 
camps. 

1  C.  H.  Gordon,  P.  P.  68,  U.  S.  G.  S.,  p.  269. 


134 


EXAMINATION  OF  PROSPECTS 


Ore -Shoots  at  the  Tops  of  Anticlines,  or  "Saddle  Reefs."— In 

some  districts  the  ore-deposits  occur  persistently  at  the  tops  of 
anticlinal  folds.  In  many  cases  this  appears  to  be  the  result  of 
local  impounding  of  ascending  solutions  at  the  tops  of  anticlines 
that  are  capped  by  strata  of  impervious  rock.  Fractures  and 
open  spaces  form  in  the  convex  side  of  folded  strata  somewhat 
before  the  production  of  a  continuous  fissure,  and  the  largest 


/         \ 


FIG.  63. — Plan  of  ore-bodies,  Tombstone,  Arizona,  showing  their  relation 
to  anticlinal  folds.     After  Church. 

open  spaces  along 'fissures  through  folded  rocks  are  often  found 
at  such  points;  these  open  spaces  appear  to  be  favorable  to  the 
segregation  of  ore  minerals.  Rising  solutions,  therefore,  are 
likely  to  form  ore-bodies  at  the  tops  of  anticlines.  The  same 
relation  appears  to  hold  true  for  the  localization  of  secondary 
ores  in  synclinal  troughs  by  descending  solutions. 

AT  TOMBSTONE,  ARIZONA/  the  deposits  lie  in  the  anticlines,  in 
some  cases  on  the  flank,  in  other  cases  at  the  apex,  but  the 
synclines  are  barren.  Ore-shoots  in  the  veins  are  associated  with 
anticlinal  folds  of  the  wall  rock. 


1  John  A.  Church,  Trans.  A.  I.  M.  E.,  XXXIII,  p.  14. 


PRIMARY  ORE-SHOOTS 


135 


AT  BENDIGO,  AUSTRALIA/  quartz  veins  traverse  highly  folded 
shales  and  sandstones.  The  ore-deposits  are  local  developments 
at  the  tops  of  the  anticlines,  and  in  typical  cases  a  series  of  these 
so-galled  "saddles"  are  superposed;  they  are  usually  connected 
by  stringers  with  underlying  and  overlying  saddles,  or  with  the 
mineralizing  fissure. 


"' 


FIG.  64.  —  Section  through  a  saddle  reef,  Bendigo,  Victoria.     A,  Sand- 
stone; B,  shaly  sandstone;  C,  quartz-ore.     After  Rickard. 

IN  THE  NOVA  SCOTIAN  GOLD  FIELDS.  —  Mr.  E.  R.  Faribault2  has 
described  a  condition  similar  to  that  at  Bendigo,  Australia. 
In  the  example  given,  of  21  arches  of  strata  on  the  anticlines, 
20  contain  gold  ore.  The  cause  of  the  localization  appears  to 


1  T.  A.  Rickard,  Trans.,  A.  I.  M.  E.,  XX,  p.  480. 

2  Trans.,  Can.  M.  I.,  1899;  Beck-  Weed,  p.  472. 


136  EXAMINATION  OF  PROSPECTS 

have  been  opening  or  parting  due  to  slipping,  as  shown  by 
slickensides,  along  the  contact  planes  between  quartzite  and 
slate.  That  the  parting  and  deposition  was  gradual  and  pro- 
gressive is  shown  by  the  crustified  structure  of  the  ore. 

AT  THE  ELKHORN  MINE,  ELKHORN,  MONTANA/  the  principal 
deposit  occurs  as  a  saddle-shaped  mass  along  the  axis  of  a 
steeply  pitching  anticlinal  fold.  The  ore  occurs  at  a  contact 
between  altered  shale  and  a  massive  crystalline  limestone. 
The  bedding  planes  are  ore  bearing  only  where  the  general  dip 
of  the  rocks  is  disturbed  by  flexures.  In  addition  to  the  ore 
formed  along  the  contact,  a  number  of  very  large  ore-bodies 
have  been  found  at  some  little  distance  from  it  in  the  dolomite, 
but  always  in  the  same  structural  position. 

The  Chemical  Influence  of  Wall  Rocks  on  Ore -Shoots.— The 
chemical  effects  of  different  wall  rocks  upon  the  segregation  of 
minerals  into  ore-shoots  is  well  established  by  many  clear 
examples. 

A  vein  that  indicates  by  a  well-developed  banded  structure 
or  crustification  its  origin  through  the  filling  of  open  spaces  is 
unlikely  to  vary  in  mineral  content  in  passing  from  one  country 
rock  into  another,  except  as  the  structural  character  of  the 
original  fissure  varies  in  the  different  rocks.  -In  veins  that 
were  formed  by  a  replacement  of  the  walls  of  their  fissures, 
however,  the  mineral  content  is  likely  to  vary  markedly  in 
different  rocks,  in  proportion  to  the  precipitative  action,  permea- 
bility, and  solubility  of  the  several  rocks  encountered. 

As  the  chemical  activity  of  any  rock  mass  must  vary  with  the 
character  of  different  mineralizing  solutions,  being  greater  or 
less  according  to  the  kinds  and  amounts  of  the  dissolved  materials, 
no  general  rule  may  be  formulated  to  apply  to  all  veins,  nor  to 
any  given  type  of  country  rock.  In  general,  however,  where  a 
vein  passes  from  a  massive  and  insoluble  rock  to  one  that  is 
relatively  pervious,  soluble,  and  replaceable,  the  probabilities 
are  that  the  usual  vein  filling  will  expand  and  a  relatively  large 
replacement  body  will  be  found  in  the  favorable  rock.  Such 

1  W.  H.  Weed,  Trans.,  A.  I.  M.  E.,  XXXI,  p.  647. 


PRIMARY  ORE-SHOOTS 


137 


enlargements  frequently  extend  outward  for  considerable  dis- 
tances from  the  mineralizing  vein,  which  may  indeed  be  an 
insignificant  and  unmineralized  fissure,  known  only  through  its 
effect  in  having  transported  solutions  to  a  rock  horizon  suitable 
for  the  precipitation  of  their  mineral  content. 

A  distinction  is  difficult  between  the  practically  very  different 
but  genetically  similar  types  whose  extremes  are  represented  by 


FIG.  65. — Section  through  the  Dolcoath  mine,  Cornwall,  showing  the 
copper  stopes  where  the  vein  is  in  slates,  and  the  tin  stopes  where  the  vein 
traverses  granite.  Ajter  LeNeve  Foster. 

replacement  deposits  on  one  hand,  and  by  typical  veins  whose 
ore-shoots  are  the  result  of  chemical  action  of  the  wall  rock  on 
the  other. 

The  relative  precipitative  action  of  the  various  rocks  upon 
mineralizing  solutions  may  not  be  stated  definitely,  but  there 
are  certain  general  relations  that  may  be  considered  well  demon- 
strated in  spite  of  exceptions.  The  sedimentary  rocks  are,  in 
general,  more  active  chemical  precipitants  and  more  easily  re- 
placeable than  are  the  igneous  rocks,  although  this  is  not  in- 


138 


EXAMINATION  OF  PROSPECTS 


variably  the  case.  Limestones  are  among  the  most  chemically 
active  rocks  in  precipitating  minerals  from  solutions,  and  are 
most  easily  replaceable  by  mineralizing  solutions  of  usual  com- 
position. Next  to  limestones  come  calcareous  shales,  sand- 
stones, quartzites  and  conglomerates. 

In  each  case  the  chemical  and  physical  character  of  the  rock 
and  of  the  mineralizing  solution  determines  the  most  favorable 
locus  for  deposition,  and  local  data,  as  usual,  is  of  more  value  in 
predicting  the  most  favorable  horizon  for  exploration  than  is 
the  best  authenticated  general  data.  Where  several  beds  of  the 


FIG.  66. — Sections  showing  the  behavior  of  veins  in  passing  through 
porphyry  dikes,  Cornwall,  England;  the  values  are  segregated  where  the 
porphyry  forms  the  walls  of  the  veins.  After  De  la  Beche. 

same  sediments  are  present,  it  is  usual  that  one  particular  bed 
has  been  the  most  active  chemically,  and  carries  the  ore-deposits 
to  the  perhaps  complete  exclusion  of  the  other  beds  of  the 
series.  Carbon  is  an  active  precipitant  of  minerals  from  solu- 
tions. The  rock  or  bed  that  will  be  most  active  chemically  in 
precipitating  minerals  and  will  contain  the  richest  or  largest 
ore-bodies  may  occasionally  be  predicted  from  its  being  known 
to  contain  carbonaceous  matter.  Most  sedimentary  beds  con- 
tain iron,  and  not  infrequently  this  combines  with  sulphur 
contained  in  carbonaceous  material  to  form  pyrite,  which  is  an 
active  precipitant  of  other  sulphides  and  of  gold.  This  con- 
dition is  typical  of  shales.  .The  influence  of  carbon,  contained 
in  wall  rocks,  upon  the  precipitation  of  ores,,  while  undoubtedly 


PRIMARY  ORE-SHOOTS  139 

the  controlling  factor  in  some  districts,  has  probably  been  over- 
rated; the  amount  and  distribution  of  carbonaceous  matter  is 
wholly  incapable  of  explaining  the  occurrences  of  many  large 
replacement  deposits;  its  chief  function  may  be,  perhaps,  to 
start  a  precipitation  which  is  carried  on  through  mass  action  in 
the  easily  replaceable  rock. 

The  selective  precipitative  action  of  different  igneous  rocks  is 
also  well  established,  but  although  the  different  effects  of  various 
rocks  in  segregating  values  may  be  apparent,  the  differing 
characteristics  that  produce  these  effects  are  not  usually  notice- 
able, and  in  advance  of  local  information,  no  one  rock  may  be 
considered  more  favorable  for  ore  deposition  than  another. 
Many  veins  pass  through  several  igneous  rocks  without  apparent 
change  in  value  or  mineral  content. 

In  the  exploration  of  a  vein  in  depth  a  cessation  of  payable  ore 
should  not  immediately  be  assigned  to  primary  impoverishment 
due  to  depth  alone,  for  it  has  been  demonstrated  that  such 
impoverishment  frequently,  or  even  usually,  takes  place  gradu- 
ally and  over  great  vertical  distances.  In  cases  where  the  segre- 
gation of  ore  into  shoots  may  be  referred  to  the  precipitative 
action  of  wall  rocks,  the  possibility  of  a  reccurrence  of  the 
favorable  wall  rock  at  greater  depths  at  once  suggests  itself.  If 
the  exposed  ore  is  due  to  geological  factors  unlikely  to  recur 
in  depth,  then  deeper  exploration  is  unwarranted;  if  the  geo- 
logical conditions  appear  to  be  as  good  in  depth  as  in  the  horizon 
of  known  productivity,  then  deeper  exploration  for  primary  ore 
may  be  justified. 

In  the  secondary  concentration  of  metals  by  surface  agencies 
the  character  of  the  country  rock  is  of  the  greatest  importance, 
some  rocks  actively  precipitating  metals  and  not  permitting  the 
migration  necessary  to  form  enrichments. 

Where  it  is  seen  that  a  vein  is  to  pass  into  a  different  rock,  the 
change*  will  be  awaited  with  anxiety  unless  it  is  known  that  the 
vein  has  been  formed  through  the  filling  of  open  spaces,  when 
a  change  is  much  less  likely  than  in  a  vein  formed  partly  by  re- 
placement. It  is  seldom  that  such  a  change  in  wall  rocks  may 


140 


EXAMINATION  OF  PROSPECTS 


be  expected  to  accompany  better  ore,  unless  the  expectation  is 
based  upon  abundant  local  evidence. 

AT  NEIHART,  MONTANA/  the  primary  ore  consists  of  galena 
zincblende  and  pyrite  in  a  gangue  of  carbonates  of  lime,  iron  and 
manganese;  the  veins  carry  ore-shoots  between  walls  of  pink  or 
white  feldspathic  gneiss,  but  are  barren  where  they  pass  through 
dark  colored  gneisses,  amphibolite  and  diorite.  The  mineralizing 
solutions  appear  to  have  reacted  with  the  feldspars  and  not 
with  the  ferro-magnesian  silicates. 


Pinto  diorite;  no  ore 


Gray  gneiss;  no  ore 


Black  gneiss;  no  ore 


x^  '  Red  gneiss;  good  ore 
^-x/ 

Dark  gray  schist; 
no  ore 

FIG.  67. — Diagram  (plan)  showing  the  rocks  traversed  by  the  Neihart  veins 
and   their   ore   content  in   the   various   rocks.     After   Weed. 

AT  BUTTE,  MONTANA/  a  series  of  copper  veins  intersect 
quartz-monzonite,  aplite,  and  quartz-porphyry.  The  veins  were 
formed  through  replacement  of  the  wall  rocks,  which  are  much 
altered-;  in  the  monzonite  they  are  commonly  rich  in  copper;  in 
the  aplite  they  are  equally  wide  and  strong,  but  are  lean,  being 
composed  chiefly  of  quartz  with  comparatively  little  pyrite 
and  copper.  In  the  porphyry  the  veins  are  narrow  and  lean. 
The  pyrite  contained  in  the  veins  where  they  pass  through 
aplite  and  the  quartz-porphyry  is  noticeably  poor  in  copper. 


1  W.  H.  Weed,  Trans.,  A.  I.  M.  E.,  XXXI,  p.  645. 

2  W.  H.  Weed,  Trans.,  A.  I.  M.  E.,  XXXI,  p.  642. 


PRIMARY  ORE-SHOOTS  141 

In  this  district  the  ore-shoots  favor  the  more  basic  rock.  The 
silver  veins  at  Butte  exhibit  a  marked  crustification,  and  are 
clearly  the  result  of  the  filling  of  open  fissures.  They  traverse 
both  the  monzonite  and  the  aplite,  but  there  is  no  perceptible 
difference  in  their  character  or  tenor  where  they  pass  through 
the  different  rocks. 


IW.V.'fl  Granite 

FIG.  68. — Ideal  plan  of  a  copper  vein,  Butte,  Montana,  showing  the 
impoverishment  where  aplite  forms  the  walls  of  the  vein;  the  ore  is  indicated 
in  black.  After  Weed. 

AT  GYMPIE,  EASTERN  AUSTRALIA/  the  precipitative  influence 
of  a  slate  band  is  so  marked  that  it  is  followed  in  exploration  in- 
stead of  the  veins.  The  main  slate  band  is  200  ft.  in  thickness, 
and  the  several  veins  carry  ore  where  they  pass  through  it,  but  are 
elsewhere  unpayable. 

AT  ASPEN,  COLORADO, 2  the  mineralizing  solutions  appear  to 
have  risen  through  the  underlying  siliceous  formations  (granite 
and  quartzite)  w^ith  but  little  precipitation  of  ores,  and  through 
the  overlying  dolomitic  and  calcareous  formations  with  much  pre- 
cipitation, and  finally,  upon  encountering  the  overlying  shales 
and  the  prophyry  sheet  that  was  intruded  at  their  base,  the 
solutions  were  impounded,  and  the  shales  reacted  powerfully 
with  the  solutions  for  the  production  of  large  deposits. 

AT  BALLARAT,  AUSTRALIA, 3  the  prevailing  country  rocks  are 

1  J.  H.  Curie,  "Gold  Mines  of  the  World,"  p.  188. 

2  J.  E.  Spurr,  Economic  Geology,  IV,  p.  310. 

2  T.  A.  Rickard,  Trans.,  A.  I.  M.  E.,  XXX,  p.  1010. 


142 


EXAMINATION  OF  PROSPECTS 


slates  and  sandstones  that  carry  as  intercalated  members  a  series 
of  black  slate  bands,  which  contain  carbonaceous  matter  and 
pyrite.  These  bands  are  persistent  over  long  distances  and 
while  typically  very  thin,  being  from,  a  fraction  of  an  inch  to  a 
few  inches  in  thickness  only,  they  have  had  a  remarkable  effect 


WfJBt 


Horizontal 


West 


Horizontal 


FIG.  69. — Plans  of  the  "Indicator"  slate  seam  near.     Ballarat,  Australia, 
showing  the  segregation  of  gold  in  the  intersecting  quartz  veins.     After  Dunn. 

in  segregating  the  gold  content  of  a  series  of  quartz  veins  that 
intersect  them.  The  chief  of  these  slate  seams  is  known  as  the 
"Indicator"  and  along  the  intersections  of  the  quartz  veins 
with  it,  as  well  as  with  the  other  seams,  the  ore-shoots  have 
formed. 


PRIMARY  ORE-SHOOTS  143 

AT  SAN  JAVIER  AND  TECORIPA,  SONORA,  MEXICO,  an  extreme 
case  of  the  precipitative  action  of  carbon  upon  mineralizing  solu- 
tions is  evident.  Here  the  solutions  have  followed  steeply  dip- 
ping and. much  altered  coal  seams;  the  carbonaceous  matter  has 
in  great  part  been  altered  to  graphite^with  which  are  associated 
the  rich  silver  ores. 

IN  THE  SAN  JUAN  MOUNTAINS,  COLORADO/  the  distribution  of 
the  primary  ore  in  veins  that  cut  a  series  of  volcanic  beds  appears 
to  be  due  to  the  greater  precipitative  effect  of  certain  beds  over 
others.  The  veins  are  commonly  low  in  grade  in  the  rhyolitic 
layers,  and  of  good  grade,  frequently  of  high  grade,  in  the  andes- 
itic  layers.  It  is  said  that  in  the  San  Juan  andesitic  breccias 
the  value  of  the  ore  varies  from  layer  to  layer.  Ore  deposition 
here  appears  to  have  been  greater  in  the  basic  than  in  the  sili- 
ceous rocks. 

IN  THE  PLATEAU  REGION  OF  ARIZONA  AND  NEW  MEXICO,  there 
are  many  districts  where  tree  trunks  contained  in  red  Triassic 
sandstones  have  by  replacement  been  transformed  wholly  or  in 
part  to  chalcocite,  which  appears  in  this  connection  to  be  a 
primary  mineral.  The  quantity  of  ore  being  limited  to  the  car- 
bonaceous material,  which  was  scanty  in  amount  and  erratic  in 
distribution,  these  tree  trunk  deposits  have  yielded  small  com- 
mercial returns. 

NEAR  SAN  BERNARDO,  SONORA,  MEXICO,  a  small  quantity  of 
high-grade  ore  was  mined  where  a  tree  trunk  had  been  replaced 
by  chalcocite  and  silver  glance.  The  whole  was  tree  extracted, 
including  several  branches,  when  the  "mine"  came  to  an  end. 

Ore -Shoots  in  Veins  of  Deep-seated  Origin. — In  many  veins  of 
deep-seated  origin  the  ore  occurs  in  well-defined  shoots  which, 
apparently,  are  not  connected  with  any  of  the  factors  discussed 
in  the  preceding  paragraphs;  to  this  class  belong  some  of  the 
most  important  ore-shoots  known. 

Inasmuch  as  a  localization  of  values  must  be  assigned  to  a 
local  difference  or  cause,  these  shoots  are  probably  best  referred 
to  either  mass  action,  or  to  a  change  in  the  predominant  circula- 

1  C.  W.  Purington,  Economic  Geology,  I,  p.  133. 


144  EXAMINATION  OF  PROSPECTS 

tion  channels  at  different  times  in  the  formation  of  the  vein  or 
deposit.  In  the  former  case,  a  precipitation  of  a  metal  once 
started,  by  any  cause — and  all  precipitations  must  have  a  local 
start —  is  continued  progressively,  the  mineral  formed  attracting 
to  itself  additions  of  the  same  kind  from  the  solutions  until  the 
result  is  a  localization  of  values,  or  an  ore-shoot.  In  the  latter 
case,  it  is  difficult  to  conceive  that  a  fissure  at  all  times  permits 
the  passage  of  solutions  over  its  entire  length  with  equal  facility; 
therefore,  at  any  given  time  certain  parts  of  a  fissure  probably 
constitute  the  main  channels  for  the  passage  of  solutions  and  so 
give  rise  to  the  localization  of  values  in  ore-shoots.  It  is  prob- 
able that  veins,  constituting  lines  of  weakness,  have  been  opened 
and  reopened  many  times  during  ore  deposition;  the  ore- 
shoots,  therefore,  may  be  considered  to  represent  the  circulation 
channels  at  the  time  of  passage  of  the  richest  solutions. 

Many  ore-shoots  are  of  lenticular  shape,  being  widest  at  the 
center  and  decreasing  in  thickness  toward  their  peripheries, 
where  they  die  out  in  stringers  and  fade  into  barren  material. 
The  study  of  fissures  themselves  indicates  for  them  a  similar 
form,  inasmuch  as  they  are  strongest  over  their  central  portions, 
and  die  out  at  their  ends  in  stringers  or  fade  out  in  the  rock  where 
the  stresses  that  formed  them  were  absorbed  without  fracture. 
This  type  of  shoot  may,  therefore,  be  referred  to  a  reopening  of  the 
vein  in  a  manner  similar  to  that  of  the  formation  of  fissures 
through  rocks. 

Certain  ore-shoots  of  great  depth  as  compared  to  width  may 
be  due  to  incipient  folds  on  the  plane  of  the  vein,  such  lines  of 
weakness  in  the  vein  as  were  most  nearly  vertical  affording  the 
most  favorable  channels  for  rising  solutions. 


CHAPTER  VII 
THE   PRIMARY   ALTERATION   OF   WALL   ROCKS 

Metamorphic  Processes. — The  term  metamorphism  as  com- 
monly used  means  any  change  in  a  rock  either  in  form  or  com- 
position, from  whatever  cause.  By  metasomatism  is  meant  a 
metamorphism  that  involves  a  change  in  the  chemical  composi- 
tion of  a  rock  by  the  addition  or  subtraction  of  substance.1 

Certain  types  of  metamorphism  are  due  to  the  action  of  miner- 
alizing solutions,  and  are  closely  connected  with  ore  deposition; 
they  form,  therefore,  valuable  guides  in  the  exploration  for  ore- 
deposits.  Other  types  of  metamorphism  are  due  to  regional 
processes,  and  are  not  connected  with  ore  deposition. 

Dynamo -regional  Metamorphism. — By  dynamo-regional  met- 
amorphism is  meant  a  primary  alteration  of  rocks  over  large 
areas  under  conditions  of  pressure,  flowage,  and  heat.  This 
process  produces  schists  and  gneisses  from  either  sediments  or 
igneous  rocks.  It  is  thought  that  there  is  little  migration  of 
minerals  or  of  elements  in  dynamo-regional  metamorphism, 
and  that  the  changes  are  due  chiefly  to  recrystallization  and 
rearrangement  of  original  minerals  in  bands  with  their  major 
axes  oriented  normal  to  the  direction  of  greatest  pressure. 
Under  these  conditions  limestones  are  commonly  altered  to 
marble,  and  sandstones  to  quartzite,  basic  igneous  rocks  to 
greenstone  or  serpentine,  while  rocks  of  normal  or  acid  com- 
position commonly  yield  the  usual  types  of  schist  or  gneiss. 
,  Dynamo-regional  metamorphism,  extending  over  large  areas, 
has  no  connection  with  mineralizing  processes^and  is,  therefore, 
no  guide  to  ore-deposits;  while  this  process  has  certain  features, 
such  as  characteristic  mineral  associations,  in  common  with 
contact  metamorphism,  the  results  of  the  two  processes  may 
usually  be  distinguished  without  difficulty. 

1  Waldemar  Lindgren,  Trans.,  A.  I.  M.  E.,  XXX,  p.  580. 
10  145 


146  EXAMINATION  OF  PROSPECTS 

Contact  Metamorphism. — By  contact  metamorphism  is  meant 
the  change  in  structure  and  composition  of  the  enclosing  rocks 
in  the  immediate  vicinity  of } and  due  to  accession  of  substance^1 
from  ;  igneous  intrusions.  The  rocks  most  effected  by  this 
process  are  limestones,  shales,  and  sandstones,  which  are  altered 
respectively  to  marble,  hornfels,  and  quartzites  with  the  further 
addition  of  new  minerals;  igneous  rocks  are  effected  to  a  less 
extent  by  contact  metamorphism,  having  themselves  been  formed 
under  igneous  conditions.  The  characteristic  contact  met- 
amorphic  minerals  are  garnet,  epidote,  wollastonite,  pyroxene, 
amphibole,  magnetite,  quartz  and  sulphides;  the  unique  feature 
of  contact  metamorphic  deposits  is  the  primary  association  of 
oxides  with  sulphides.2  While  contact  metamorphism  is 
frequently  associated  with  ore  deposition,  the  development  of  a 
large  zone  of  contact  minerals  does  not  presuppose  or  indicate 
the  existence  of  ore-deposits. 

The  distance  to  which  contact  metamorphism  effects  the 
altered  rocks  varies  greatly  in  different  occurrences,  and  also  in 
different  beds  of  a  sedimentary  series,  certain  beds  being  altered 
for  long  distances  from  the  contact  while  others  may  persist 
unchanged  up  to  within  a  short  distance  of  the  intrusive.  Con- 
tact metamorphic  minerals  and  products  are  commonly  tough 
and  resistant  to  erosion,  and  are,  therefore,  likely  to  form  prom- 
inent outcrops. 

Hydrothermal  Metamorphism.3 — By  hydrothermal  metamor- 
phism is  meant  the  alteration  of  rocks  by  hot  ascending  solutions; 
it  is  distinctly  a  metasomatic  process  and  is  a  frequent  accom- 
paniment of  ore  deposition. 

The  walls  of  veins  are  commonly  fractured  or  strained  by  the 
stresses  that  produced  their  fissures,  especially  the  hanging  walls, 
and  in, this  way  ready  access  is  afforded  to  the  solutions  that 
accomplish  their  alteration.  The  alteration  is  usually  intense 

1  Waldemar  Lindgren,  P.  P.  43,  U.  S.  G.  S.,  p.  124. 

2  Waldemar  Lindgren,  Trans.,  A.  I.  M.  E.,  XXXI,  p.  227. 

3  The  present  knowledge  of  metasomatic  replacement  and  of  hydrothermal 
metamorphism  is  chiefly  due  to  Mr.  Waldemar  Lindgren. 


THE  PRIMARY  ALTERATION  OF  WALL  ROCKS         147 

immediately  along  the  veins,  but  is  likely  to  diminish  rapidly  in 
degree  with  distance  from  them.  It  appears  that  the  walls  of  a 
vein  tend  to  keep  within  them  the  heavy  bases,  and  gangue 
minerals,  but  permit  the  passage  of  the  depleted  solutions  that 
effect  the  rock  alteration.  Not  infrequently,  disseminated 
sulphides  are  found  in  the  altered  wall  rocks,  although  these 
minerals  commonly  carry  low  values  in  gold  and  silver  as  com- 
pared with  the  sulphides  deposited  within  the  veins.  Pyrite  is 
the  most  widely  disseminated  sulphide  in  wall  rocks,  and  is 
frequently  the  result  there  of  the  alteration  of  the  rock  minerals 
themselves,  in  which  case  it  does  not  carry  values  in  the  precious 
metals.  Zincblende  and  galena  are  more  rarely  found  as 
disseminations. 

v  The  results  of  hydrothermal  metamorphism  are  frequently 
valuable  guides  in  the  search  for  ore-deposits,  especially  where 
they  are  confined  to  the  vicinity  of  the  veins  or  stocks.  In 
many  cases,  however,  and  especially  where  mineralization  and 
attendant  primary  rock  alteration  have  taken  place  at  slight 
depth  below  the  surface,  the  porosity  and  shattered  condition 
of  the  wall  rocks  has  permitted  a  wide  distribution  of  the  alter- 
ing waters,  and  the  altered  areas  are  likely  to  be  large,  and  so  to 
lose  their  value  as  aids  in  the  search  for  ore-bodies. 

In  many  parts  of  the  arid  regions  of  the  western  United  States 
and  Mexico  there  are  large  areas  of  brilliant  red  hills,  the  rocks  of 
which  have  suffered  hydrothermal  metamorphism;  the  iron 
mineralization  of  these  red  hills  is  likely  to  be  more  apparent  than 
real,  the  surface  being  stained  a  deep  red,  yellow  or  brown,  but 
upon  fresh  fractures  the  rock  is  seen  to  carry  little  iron,  present 
in  most  cases  as  pyrite,  resulting  from  the  alteration  of  the  rock 
minerals. 

The  alteration  of  wall  rocks  is  likely  to  be  less  along  deep- 
seated  veins  than  in  deposits  of  shallower  origin;  it  is  also  likely 
to  reach  a  less  development  where  it  accompanies  deposits 
resulting  from  the  filling  of  open  spaces  than  in  connection  with 
replacement  deposits. 

The  most  common  effect  of  intense  alteration  immediately 


148  EXAMINATION  OF  PROSPECTS 

along  veins  is  sericitization  and  the  introduction  of  pyrite,  which 
alteration  is  likely  to  grade  into  propylitization  with  greater 
distance  from  the  veins,  the  latter  process  being  the  less  intense 
and  more  widespread;  in  basic  rocks,  propylitization  is  likely 
to  persist  close  up  to  the  veins,  there  to  give  way  perhaps  to 
sericitization.  While  the  degree  and  extent  of  alteration  by  one 
set  of  solutions  is  likely  to  vary  markedly  in  different  rocks,  it 
not  infrequently  happens  that  the  final  products  of  alteration 
and  recrystallization  are  similar,  and  it  becomes  a  matter  of 
difficulty  to  distinguish  one  rock  from  another.  The  chief 
chemical  effects  of  hydrothermal  metamorphism  are  an  increase 
in  potash  with  attendant  loss  in  soda,  silicification,  and  the 
introduction  of  sulphides,  chiefly  pyrite. 

The  effects  of  hydrothermal  metamorphism  may  usually  be 
distinguished  readily  from  the  effects  of  hydro-metamorphism, 
or  the  alteration  accomplished  by  surface  waters:  the  latter 
alteration  is  rather  more  likely  to  be  uniform,  and  is  essentially 
a  process  of  oxidation,  hydration,  and  solution,  while  the  former 
is  characterized  by  sericitization  and  silicification.  Kaolin  is 
the  chief  guide  in  making  this  distinction,  being  a  typically 
surface  product,  often  formed  from  sericite  as  well  as  directly 
from  feldspars  through  the  action  of  sulphuric  acid  solutions  set 
free  by  the  oxidation  of  pyritic  sulphides. 

Propylitization. — By  propylitization  is  meant  a  hydrothermal 
metamorphism  that  involves  the  development  of  chlorite,  epidote 
and  pyrite  from  the  dark  rock-making  silicates,  and  of  quartz, 
calcite,  and  epidote  from  the  feldspars.  Rocks  that  have  been 
subj  ected  to  propylitic  alteration  are  commonly  greenish-gray  in 
color  and  carry  bright  green  stains  of  epidote;  the  barren  pyrite 
developed  is  present  in  well-developed  crystals,  and  upon  oxida- 
tion is  likely  to  color  the  surface  red,  brown  or  yellow. 
v  This  type  of  rock  alteration  is  frequently  present  over  large 
areas  in  mineralized  districts,  and  is  rarely  a  guide  to  ore-deposits, 
except  as  indicating  in  general  that  a  district  has  been  subjected 
to  metamorphic  action. 

Propylitization    is    thought  to  have  taken  place  at  no  great 


THE  PRIMARY  ALTERATION  OF  WALL  ROCKS         149 

depth  beneath  the  surface,  where  extensive  fracturing  and 
jointing  permitted  easy  access  by  solutions,  and  although 
associated  in  a  general  way  with  ore  deposition,  is  rarely  to 
be  referred  to  any  particular  fissure  or  series  of  fissures.  In 
rocks  that  have  suffered  propylitic  alteration  the  feldspars  have 
usually  become  dull,  if  not  more  completely  altered,  but  the 
principal  physical  features  of  the  rocks,  such  as  texture,  are  pre- 
served. Propylitic  alteration  is  most  prominently  developed 
in  rocks  of  intermediate  or  basic  composition. 

Sericitization. — By  sericitization  is  meant  a  hydrothermal 
metamorphism  that  results  in  an  almost  complete  loss  of  soda 
and  a  large  gain  in  potash,  silica,  and  commonly  also  of  pyrite, 
with  occasionally  lesser  quantites  of  carbonic  acid  and  fluorine.,-^ 
The  typical  result  of  complete  sericitization  is  a  finely  granular 
aggregate  of  sericite,  quartz,  pyrite  and  calcite;  after  thorough 
sericitization  it  is  difficult  to  distinguish  between  rocks  that 
were  originally  quite  dissimilar. 

Sericitization  is  a  common  form  of  hydrothermal  metamor- 
phism, and  is  an  intimate  accompaniment  of  the  mineralization 
of  most  ore-deposits  in  igneous  rocks;  the  alteration  is  commonly 
most  intense  immediately  along  the  veins,  and  decreases  with 
distance  from  them,  sometimes  passing  into  a  propylitic  altera- 
tion, which  latter  transition  is  likely  to  take  place  within  a 
short  distance  of  the  veins  in  basic  rocks;  sericitization  not  in- 
frequently persists  over  large  areas  in  rocks  of  intermediate  or 
acid  composition. 

V  Sericitized  rocks  are  commonly  white  to  light  yellow  in  color, 
and  are  usually  soft;  the  minute  scales  or  plates  of  sericite  are 
usually  distinguishible  in  hand  specimens  and  often  form  a 
means  of  distinguishing  a  sericitized  from  a  kaolinized  rock; 
the  two  minerals  are  likely  to  occur  together,  however,  the  latter 
being  formed  from  the  former  as  well  as  from  feldspars  by  the 
action  of  surface  waters  carrying  sulphuric  acid.  The  pyrite  in- 
troduced in  sericitization  is  sometimes  accompanied  by  minute 
quantities  of  chalcopyrite  and  zincblende,  less  frequently  by 
galena  or  arsenopyrite. 


150  EXAMINATION  OF  PROSPECTS 

AT  BISBEE,  ARIZONA.1 — The  porphyry  of  Sacramento  Hill  and 
the  adjacent  schists  have  undergone  sericitic  alteration.  The 
results  of  alteration  of  both  rocks  are  similar.  The  original 
quartz  has  recrystallized  into  quartz  aggregates,  the  feldspars 
have  become  areas  of  sericite,  quartz  and  pyrite,  with  occasion- 
ally a  little  epidote,  chlorite,  zircon  and  rutile.  Under  the 
action  of  surface  agencies  the  pyrite  has  oxidized,  and  much  of 
the  sericite  has  been  altered  to  kaolin.  The  surface  rock  con- 
sists chiefly  of  an  aggregate  of  quartz,  stained  and  streaked 
by  iron,  and  containing  nests  of  sericite  and  kaolin  at  the  surface. 
Much  of  the  kaolin  has  weathered  out,  leaving  cavities.  The 
outcrops  are  brownish-red  in  color,  but  the  rock  in  most  ex- 
posures is  white  on  fresh  fractures. 

Alunitic  Alteration.2 — The  occurrence  of  alunite  as  a  primary 
mineral  has  been  established  at  Goldfield,  Nevada.  It  is  here 
associated  with  gold,  sulphides,  tellurides,  sulphantimonites,  and 
kaolin,  as  a  primary  result  of  hydrothermal  metamorphism. 
Kaolin  is  elsewhere  regarded  as  a  typical  product  of  the  action 
of  surface  waters.  The  alunitic  alteration  at  Goldfield  is  wide- 
spread, and  is  intimately  associated  with  ore  deposition.  Alunite 
occurs  massive,  as  a  crystalline  constituent  of  the  altered  rocks 
in  the  ore-bearing  areas,  and  intercrystallized  with  pyrite,  gold, 
tellurides  and  other  minerals  in  the  ores. 

Alteration  to  Greisen  Along  Tin  Veins. — The  wall  rocks  of  tin 
veins  are  characteristically  altered  by  hydrothermal  metamor- 
phism to  greisen,  the  alteration  consisting  of  the  replacement  of 
feldspars  and  of  biotite  by  quartz,  lepidolite,  topaz,  tourmaline, 
and  fluorite,  commonly  associated  with  cassiterite,  wolframite, 
chalcopyrite  and  arsenopyrite.  The  altered  rock  in  this  case 
frequently  constitutes  payable  ore.  Greisen  is  commonly 
referred  to  as  altered  granite.  Where  the  altered  walls  are  other 
rocks,  the  same  minerals  are  likely  to  be  developed. 

Silicification. — Silicification  of  wall  rocks  by  hydrothermal 
metamorphism  is  a  common  form  of  primary  alteration  that  is 

1  F.  L.  Ransome,  P.  P.  21,  U.  S.  G.  S.,  p.  150. 

2  F.  L.  Ransome,  P.  P.  66,  U.  S.  G.  S.,  p.  192. 


THE  PRIMARY  ALTERATION  OF  WALL  ROCKS         151 

frequently  associated  with  ore  deposition.  The  tendency  toward 
silicification  is  probably  greater  in  acid  than  in  basic  rocks,  but 
the  process  is  common  also  in  limestones,  where  it  is  likely  to 


FIG.  70. — Diagram  showing  the  alunitized  areas  and  the  known  productive 
areas  at  Goldfield,  Nevada.     After  Ransome. 

form  a  reticulated  structure,  siliceous  seams  enclosing  partially 
altered  or  fresh  fragments.  Quartz  formed  in  this  manner  is 
commonly  of  the  cherty  variety;  the  replacement  of  calcite  by 
silica  often  reproduces  closely  the  original  structure,  where  the 


152  EXAMINATION  OF  PROSPECTS 

difference  in  hardness  between  the  altered  and  the  unaltered 
parts  offers  the  best  means  of  distinction. 

Silicification  frequently  accompanies  other  forms  of  hydro- 
thermal  metamorphisnf,  where  it  is  likely  to  represent  the  ex- 
treme of  alteration,  and  thus  serve  as  a  valuable  guide  in  a 
search  for  ore-deposits. 

Marmorization. — The  alteration  of  limestone  to  marble  is 
frequently  accomplished  by  contact  and  by  hydrothermal  as  well 
as  by  regional  metamorphism.  A  large,  thoroughly  marmor- 
ized  area  is  probably  without  significance  of  mineralization,  but 
local  or  irregular  marmorization  along  contacts  with  a  mineral- 
izing intrusive,  or  along  fissures,  is  not  infrequently  associated 
with  ore  deposition. 

Dolomitization. — The  replacement  of  a  part  of  the  calcium  in 
limestone  by  magnesium  is  called  dolomitization,  and  the 
resulting  rock,  dolomite.  This  change  is  commonly  effected  over 
large  areas,  and  has  no  connection  with  ore  deposition.  Dolo- 
mitization is  occasionally,  however,  of  local  occurrence,  and  as 
such  may  in  some  cases  be  the  accompaniment  of  ore  deposition. 
This  alteration  is  accompanied  by  an  important  shrinkage  in 
volume,  and  may  thus  indirectly  play  a  part  in  ore  deposition. 


CHAPTER  VIII1" 
ALTERATIONS  BY  SURFACE  AGENCIES 

By  hydrometamorphism  is  meant  the  alteration  of  rocks,  ores 
and  minerals  by  atmospheric  waters.  In  its  broadest  sense,  it 
includes  the  varied  processes  of  weathering,  oxidation,  hydration, 
the  leaching  of  rocks  and  ores,  and  the  solution,  migration  and 
enrichment  of  metals. 

Rain  water,  as  it  strikes  the  earth,  is  practically  pure  water, 
except  for  small  quantities  of  dissolved  atmospheric  gases;  upon 
coming  in  contact  with  ore  minerals,  especially  sulphides,  how- 
ever, it  is  transformed  into  solutions  of  great  chemical  activity, 
which  attack,  transform  and  rearrange  the  minerals  of  ore- 
bodies,  and  so  effect  enrichments  of  the  greatest  economic 
importance.  Primary  deposits  so  low  in  grade  as  to  be  without 
commercial  value  are  frequently  thus  transformed  into  deposits 
of  commercial  importance.  It  is  necessary  in  the  examination 
of  any  ore-body,  therefore,  to  determine  whether  the  valuable  ore 
is  of  primary  or  of  secondary  origin;  if  primary,  its  values  may 
be  expected  to  continue  indefinitely  in  depth  to  the  zone  of  primary 
impoverishment;  if  secondary,  the  values  are  known  to  be  con- 
trolled by  surface  agencies,  and  the  valuable  ore  may  be  expected 
to  continue  to  such  depths  only  as  have  been  reached  by  surface 
waters. 

The  Decomposition  and  Weathering  of  Rocks. — The  decomposi- 
tion and  weathering  of  rocks,  unlike  the  changes  undergone  by 
ore-bodies,  are  accomplished  by  water  carrying  as  dissolved  con- 
stituents chiefly  atmospheric  oxygen  and  carbon  dioxide,  to- 
gether with  vegetable  acids.  The  chemical  changes  wrought 
are  principally  hydration,  oxidation  and  the  formation  of  car- 
bonates. The  tendency  is  toward  the  solution  of  the  more 
easily  dissolved  minerals,  which  is  attended  by  the  formation 
and  a  surface  concentration  of  the  more  resistant  minerals, 

153 


154  EXAMINATION  OF  PROSPECTS 

among  which  are  quartz,  aluminous  clays  and  limonite;  complex 
minerals  are  thus  broken  down  into  simpler  constituents  more 
resistant  to  weathering,  which  remain  at  the  surface. 

The  weathering  of  rocks  is  characteristically  regular  over  large 
areas,  and  is  rarely  mistaken  for  the  results  of  hydrothermal 
alteration  or  kaolinization.  The  residual  minerals  from  rock 
decomposition  are  commonly  soft,  and  are  readily  carried  away 
by  erosion,  which  is  likely  constantly  to  present  fresh  rock  sur- 
faces to  the  attack  of  the  chemical  alteration  above  outlined. 
Even  in  regions  of  heavy  rainfall,  however,  a  combination  of 
slight  slopes  and  abundant  and  active  vegetable  acids  may  result 
in  deep  accumulations  of  residual  minerals,  as  frequently  occurs 
in  tropical  countries. 

Weathering  attended  by  slight  or  partial  decomposition  results 
chiefly  in  the  staining  of  the  surface  rock  red,  brown  or  yellow 
by  iron  oxides  set  free  by  the  alteration  of  basic  silicates,  or 
magnetite,  and  in  the  alteration  of  amphibolitic  rocks  to 
serpentine. 

Where  atmospheric  waters  percolate  to  depths  considerably 
below  the  surface,  their  oxidizing  power  is  diminished,  and  hydra- 
tion  is  the  principal  alteration;  the  minerals  thus  formed  through 
rearrangement  of  original  constituents,  and  to  a  less  extent 
through  introduced  substances,  are  epidote,  chlorite,  serpentine, 
pyrite)  zeolites  and  quartz. 

Leaching. — Leaching,  usually  accompanied  by  kaolinization, 
is  a  process  of  solution  exercised  by  surface  waters  by  which  the 
soluble  minerals  are  removed  and  the  resistant  minerals  left  as 
residual  products.  The  minerals  resistant  to  leaching  and  which 
commonly  make  up  the  leached  upper  parts  of  ore-bodies  are 
quartz,  kaolin,  limonite  and  oxide  of  manganese.  The  com- 
pleteness with  which  minerals  are  removed  from  the  leached 
zone  depends  upon  their  relative  solubilities  and  also  upon  the 
presence  or  absence  of  effective  precipitants  in  that  zone,  which 
latter  may  cause  residual  masses  of  oxidized  ores  to  form  in  the 
leached  zone  at  the  expense  of  the  enrichments  below. 

In   many  ore-bodies  the  presence    of    active   oxidizing  pre- 


ALTERATIONS  BY  SURFACE  AGENCIES  155 

cipitants  causes  the  metals  to  be  precipitated  immediately  or 
shortly  after  undergoing  solution,  a  process  known  as  pre- 
cipitation in  situ;  where  this  process  predominates,  secondary 
enrichments  are  prevented  or  are  of  relatively  slight  importance. 

Under  certain  conditions  limonite  is  completely  removed,  and 
the  leached  zone,  white  in  color,  is  made  up  of  quartz,  koalin 
and  unchanged  sericite.  Where  oxidation  and  solution  of  the 
accessory  minerals  proceed  more  rapidly  than  the  solution 
and  migration  of  the  valuable  metal,  as  often  happens  in  deposits 
containing  gold,  the  reduction  in  mass  may  result  in  a  residual 
concentration;  this  form  of  enrichment  is  not  uncommon,  and 
is  very  different  from  the  secondary  enrichment  due  solution  and 
migration. 

Kaolinization. — Kaolinization  is  typically  the  result  of  surface 
agencies,  except,  perhaps,  where  hydrothermal  solutions  were 
acid  in  character.  This  process  consists  in  the  decomposition, 
and  solution  of  feldspars  and  other  minerals  with  the  formation 
of  kaolin  as  the  residual  product.  A  thorough  kaolinization 
destroys  the  original  character  of  the  rocks  attacked,  and  diverse 
rocks  upon  kaolinization  yield  products  so  similar  that  their 
original  nature  is  distinguishable  with  difficulty,  if  at  all.  This 
process  of  alteration  appears  to  depend  upon  the  acid  solutions 
set  free  by  the  oxidation  of  pyritic  sulphides. 

The  presence  of  any  considerable  amount  of  kaolin  is  likely 
to  indicate  that  sulphides  have  existed  in  the  kaolinized  area 
'and  that  they  have  been  removed  in  solution,  perhaps  with  the 
formation  of  enriched  sulphides  in  depth;  kaolinization,  there- 
fore, often  constitutes  a  valuable  guide  in  a  search  for  secondary 
ore-deposits. 

Theoretically,  kaolinization  is  attended  by  a  shrinkage  in 
volume  of  from  12  to  15  per  cent,  which  is  partly  offset  by  in- 
creased porosity.  Kaolinization  readily  attacks  feldspars,  of 
which  the  soda-lime  varieties  appear  to  be  the  most  susceptible, 
and  microcline  the  most  resistant;  sericite  also  is  readily  decom- 
posed, and  dark  silicates  are  bleached  through  the  removal  of  iron. 

The  typical  product  of  complete  kaolinization  is   a  white, 


156  EXAMINATION  OF  PROSPECTS 

soft,  earthy  mass,  in  which  no  minerals  may  be  distinguished 
by  the  unaided  eye  except  kaolin,  quartz  and  limonite,  and  in 
which  the  structure  of  the  original  rock  is  completely  destroyed. 
A  rock  that  exhibits  the  outlines  of  the  feldspar  phenocrysts 
cannot  be  considered  completely  kaolinized,  and  such  incom- 
plete alteration  rarely  overlies  important  bodies  of  enriched 
sulphides.  Kaolinization  rarely  extends  into  the  wall  rocks  of 
veins,  except  where  these  rocks  themselves  originally  carried  a 
sulphide  mineralization. 

Oxidation. — Oxidation  changes  sulphides,  arsenides,  antimo- 
nides,  tellurides  and  allied  compounds  into  oxides,  native  metals, 
and  salts  containing  oxygen,  which,  according  to  conditions 
of  solubility  and  local  precipitants,  either  remain  in  situ,  or 
migrate  to  be  redeposited  under  favorable  conditions  lower 
down  in  the  deposit. 

Access  by  oxidizing  solutions  is  the  controlling  factor  in  this 
process,  which  reaches  its  most  complete  development  in  deposits 
carrying  abundant  sulphides,  which  through  solution  and 
removal,  become  progressively  more  porous  under  the  action 
of  the  solutions  and  so  help  to  bring  the  process  to  completion. 
Sulphides  that  are  locked  up  as  small  grains  within  massive 
gangue  minerals  are  rarely  altered  by  oxidation,  unless  intense 
post-mineral  shattering  affords  access  to  the  solutions. 

Oxidation  tends  to  obscure  primary  structural  relationships, 
and  also  to  segregate  the  newly  formed  minerals  into  larger 
masses,  either  of  enriched  oxides  or  enriched  sulphides. 

Oxidation,  through  the  more  ready  circulation  of  solutions,  is 
likely  to  proceed  to  greater  depths  along  veins  than  through 
the  mass  of  the  enclosing  rocks;  the  country  rock,  therefore,  is 
likely  to  exhibit  signs  of  oxidation  in  the  vicinity  of  veins,  which 
is  a  valuable  guide  in  underground  exploration. 

The  order  in  which  the  various  sulphides  are  attacked  by 
oxidation  varies  according  to  their  relative  abundance,  and 
also  according  to  structural  conditions  and  the  character  of 
associated  minerals:  Weed  states1  that  the  general  order  of 

1  Trans.,  A.  I.  M.  E.,  XXX,  p.  429. 


ALTERATIONS  BY  SURFACE  AGENCIES  157 

attack  by  oxidation  is  directly  as  the  relative  affinities  of  the 
several  metals  for  oxygen,  and  inversely  as  their  affinities  for 
sulphur.  The  order  in  which  the  common  primary  sulphides 
are  attacked,  therefore,  is  arsenopyrite,  pyrite,  chalcopyrite, 
zincblende  and  galena. 

Chalcopyrite  in  certain  occurrences  is  quite  resistant  to  oxida- 
tion, and  pyrite  varies  in  this  respect  markedly  in  different  local- 
ities, occasionally  remaining  quite  fresh  for  many  years  in 
mine  dumps  that  are  completely  permeated  by  sulphate  solutions, 
while  elsewhere  it  has  so  strong  an  affinity  for  oxygen  as  to 
become  highly  heated  when  wetted.  Tellurides  appear  to  be 
ready  subjects  of  oxidation.1  Among  the  secondary  sulphides, 
chalcocite  is  readily  dissolved  by  acid  solutions,  as  is  illustrated 
in  the  various  leaching  processes.  Tetrahedrite  is  a  complex 
mineral  of  variable  composition,  and  is  irregular  in  its  suscep- 
tibility to  oxidation  and  solution.  Most  silver  minerals  are 
readily  attacked  by  sulphate  solutions,  but  become  fixed  in  the 
presence  of  chlorine  or  its  compounds. 

The  results  of  oxidation  are  likely  to  differ  widely  according 
to  the  mineral  associations  present,  and  are  to  a  large  extent  sub- 
ject to  the  influence  of  the  enclosing  rocks;  deposits  in  lime- 
stone are  likely  to  contain  carbonates  as  oxidized  ores,  while 
silicates  or  oxides  are  likely  to  predominate  in  acid  rocks.  Dur- 
ing the  oxidation  of  sulphide  deposits  in  limestone,  acid  solu- 
tions reacting  with  this  rock  produce  gypsum,  a  relatively 
soluble  but  strongly  combined  compound  that  resists  change  or 
precipitation;  it  is  probable  that  the  soluble  products  of  oxida- 
tion that  are  not  precipitated  leave  but  little  trace  for  present 
observation.  1  Contact  minerals  are  quite  resistant  to  the  attack 
of  oxidizing  solutions,  and  frequently  enclose  unaltered  sul- 
phides at  the  surface,  while  associated  bodies  of  originally  more 
compact  sulphides  are  oxidized  to  great  depths.  V/The  oxidation 
of  sulphide  deposits  in  limestone  is  likely  to  be  quite  irregular, 
and  to  effect  far-reaching  rearrangements  of  minerals,  as  this 
rock  is  soluble  and  permits  surface  waters  to  form  replacement 

1  Waldemar  Lindgren,  P.  P.  54,  U.  S.  G.  S.,  p.  200. 


158  EXAMINATION  OF  PROSPECTS 

deposits  to  an  extent  not  approached  in  igneous  rocks  or 
quartzite. 

A  zone  of  rich  oxidized  ore  is  not  uncommon  immediately 
above  the  zone  of  sulphide  enrichment,  having  been  formed 
through  the  oxidation  of  the  upper  part  of  that  zone.  Not 
infrequently,  and  especially  in  desert  regions,  a  deep  zone  is 
formed  of  partly  oxidized,  partly  enriched  sulphide,  ore,  which 
in  most  instances  is  not  underlain  by  any  definite  zone  of  sul- 
phide enrichment.  In  ores  of  the  base  metals  an  association  of 
oxides  with  sulphides  must  usually  be  of  smelting  grade,  as 
their  widely  differing  specific  gravities  render  concentration 
difficult. 

The  minerals  that  result  from  the  oxidation  and  hydration  of 
primary  minerals  are:1  Chalcedony,  opal,  kaolin,  limonite, 
pyrolusite,  hematite,  cuprite,  the  sulphates,  carbonates,  phos- 
phates, silicates,  and  arsenates  of  the  heavy  metals,  chlorides, 
and  native  gold,  silver  and  copper. 

VxThe  Migration  and  Enrichment  of  Metals. — During  the  oxida- 
tion of  a  deposit  where  one  or  more  metals  go  into  solution 
under  conditions  that  permit  migration,  the  metals  commonly 
descend  to  the  water  level,  where  they  are  precipitated  as  second- 
ary sulphides.  This  process  of  enrichment  is  probably  most 
thorough  and  accomplishes  its  most  important  results  in  deposits 
whose  primary  ore  consists  in  part  of  pyritic  sulphides,  which 
upon  oxidation  yield  solutions  of  sulphates  and  free  sulphuric 
acid  to  continue  the  solution  and  rearrangement.  In  deposits 
that  contain  important  quantities  of  heavy  sulphides  the  lower 
limit  of  the  zone  of  enrichment  is  commonly  well-defined. 

The  completeness  of  secondary  concentrations  of  metals  is 
usualfy  proportional  to  the  relative  solubilities  of  their  sulphides. 
Galena  is  relatively  insoluble,  and  commonly  does  not  migrate 
far,  while  copper,  whose  sulphate  is  readily  soluble,  frequently 
migrates  through  important  vertical  distances,  and  is  commonly 
concentrated  in  well-defined  zones. 

The    actual    depths    of    secondary    enrichments,    and    their 

1  Waldemar  Lindgren,  Economic  Geology,  II,  p.  124. 


ALTERATIONS  BY  SURFACE  AGENCIES  159 

relative  depths  in  different  parts  of  the  same  deposit,  are  deter- 
mined by  the  position  and  character  of  the  circulation  channels 
and  by  the  water  level.  Along  the  main  channels  of  under- 
ground water  circulation  oxidizing  and  enriching  solutions  often 
penetrate  to  considerable  depths  below  the  ground-water  level. 

The  waters  that  accomplish  solution  and  enrichment  of  metals 
are  commonly  acid  in  character,  and  where  observed,  owe  their 
activity  to  contained  sulphuric  acid  and  sulphates  of  iron. 
There  is  no  lack  of  data,  however,  to  show  that  most  of  the 
common  sulphides  are  soluble  in  pure  water,  whose  solvent 
action  is  greatly  increased  by  small  amounts  of  dissolved  salts 
or  acids,  prominently  among  which  may  be  mentioned  carbonic 
acid,  which  in  solution  attacks  pyritic  sulphides. 

Discussions  of  the  reactions  of  solution  and  precipitation  are 
of  uncertain  practical  value,  on  account  of  the  complex  and 
constantly  changing  conditions  under  which  they  take  place. 

The  character  of  the  natural  waters  that  flow  or  seep  through 
the  rocks  is  frequently  a  criterion  upon  which  to  base  an  opinion 
in  regard  to  the  probability  of  secondary  enrichments  in  depth. 
Where  the  mine  water  is  acid,  or  where  it  carries  dissolved  metals, 
the  conditions  are  known  to  be  those  under  which  migration  and 
enrichment  take  place:  where  the  mine  waters  contain  no  acid  or 
dissolved  metals,  and  where  there  is  evidence  of  precipitation 
in  situ,  there  is  but  little  likelihood  of  the  existence  of  secondary 
enrichments  in  depth.  Where  water  seeps  through  the  rocks 
and  appears  in  the  workings,  samples  may  be  gathered  for 
testing;  where  the  rate  of  evaporation  is  greater  than  the  seepage, 
efflorescences  form  on  the  walls  of  workings  that  may  likewise 
be  tested.  Effloresences  of  the  sulphates  of  iron,  copper,  zinc, 
magnesium,  sodium  and  aluminum  are  of  common  occurrence 
on  the  walls  of  mine  workings,  especially  in  arid  climates. 

Pyrite  and  other  primary  sulphides  are  probably  the  most 
active  precipitants  of  secondary  minerals.  The  theoretical 
order  in  which  metals  are  precipitated  by  sulphides  from  sul- 
phate solutions  is  directly  as  their  relative  affinities  for  sulphur. 
The  relative  affinities  of  the  common  metals  for  sulphur  is, 


160  EXAMINATION  OF  PROSPECTS 

according  to  Schuermann,  mercury,  silver,  copper,  bismuth, 
cadmium,  antimony,  tin,  lead,  zinc,  nickel,  cobalt,  iron,  arsenic 
and  manganese.  In  a  secondary  rearrangement  of  a  primary 
ore  containing  silver,  copper,  lead  and  zinc  therefore  the  second- 
ary sulphides  should  be  arranged  in  the  following  order;  upper- 
most, argentite,  then  chalcocite,  bornite,  chalcopyrite,  galena, 
zincblende,  and  at  the  bottom  pyrite.1  The  relative  solubilities 
of  the  sulphates  of  these  metals  usually  interfere  with  such  an 
arrangement;  lead  sulphate,  for  example,  is  relatively  insoluble, 
and  commonly  does  not  migrate  far  before  being  precipitated, 
while  the  more  soluble  metals  descend  farther  before  being  precipi- 
tated. Organic  matter  is  an  active  precipitant  of  metallic  salts, 
and  may  be  important  as  such  in  some  deposits:  precipitation  by 
kaolin  through  adsorption  appears  also  to  be  an  important 
process  in  the  formation  of  secondary  ores. 

''  Upward  Migration. — The  upward  migration  of  solutions  due 
to  capillary  action  is  occasionally  a  factor  in  the  rearrangement 
of  values  in  the  upper  parts  of  ore-deposits.  Limonite  is  some- 
times formed  at  the  surface  through  the  oxidation  and  evaporation 
of  solutions  carrying  sulphate  of  iron,  and  occurrences  are  known 
where  the  chloride  of  silver  owes  its  distribution  to  upward 
migration. 

'  AT  LEADVILLE,  COLORADO. 2 — At  a  point  where  an  ore-body  had 
been  to  a  large  extent  removed  by  erosion  the  loose  wash  for  a 
distance  of  10  ft.  above  the  solid  rock  was  found  to  carry  chloride 
of  silver;  several  hundred  tons  of  this  material  were  shipped 
that  carried  from  40  to  60  oz.  of  silver.  This  deposit  was  found 
at  a  depth  of  150  ft.  below  the  surface. 

The  Influence  of  Circulation  Channels  on  Secondary  Altera- 
tions.— Secondary  alterations  are  in  every  case  controlled  by 
the  channels  that  permit  surface  waters  to  circulate  through 
the  rocks  and  ores,  ia  massive  ores  in  districts  where  heavy 
rainfall  and  steep  slopes  give  rise  to  an  erosion  more  rapid  than 
percolation  and  alteration,  these  processes  are  of  no  effect,  and 

1  Waldemar  Lindgren,  P.  P.  43,  U.  S.  G.  S.,  p.  182. 

2  Max  Boehmer,  Economic  Geology,  III,  p.  340. 


ALTERATIONS  BY  SURFACE  AGENCIES  161 

primary  ores  and  minerals  appear  at  the  surface.  Where  post- 
mineral  fracturing  permits  surface  waters  to  pass  through  the 
ores,  however,  a  thorough  rearrangement  of  the  minerals  and 
values  may  be  accomplished,  even  in  districts  of  heavy  rainfall, 
steep  slopes,  and  rapid  erosion.  Solution  is  a  characteristic 
feature  of  the  action  of  surface  waters  upon  the  upper  parts  of 
ore-bodies,  which  commonly  become  more  and  more  porous  under 
their  attack  and  through  seepage  a  supply  of  water  is  thus  main- 
tained under  hydrostatic  pressure  for  the  continuance  of  the 
process,  even  in  districts  of  scanty  rainfall.  It  is  thus  seen  that 
conditions  of  heavy  or  slight  rainfall,  with  attendant  rapid  or 
slow  erosion,  may  be  balanced  or  counteracted  by  the  presence 
of  good  or  poor  circulation  channels,  and  enrichments  may 
form  under  either  set  of  conditions.  Under  conditions  of  heavy 
rainfall,  sulphide  enrichments  are  likely  to  form  without  import- 
ant zones  of  oxidized  ores;  under  desert  conditions,  the  oxidized 
zone  is  likely  to  be  deep  and  important  though  characteristically 
irregular,  often  merging  irregularly  into  the  sulphide  enrich- 
ment. The  subject  of  circulation  channels  will  be  further  dis- 
cussed in  the  chapter  on  secondary  ores  and  ore-shoots. 

The  Relation  of  Oxidation  and  Enrichment  to  Topography  and 
Water  Level. — A  close  relation  exists  between  the  depth  of  second- 
ary alteration  and  the  ground  water  level.  Oxidation  com- 
monly reaches  to  or  somewhat  below  the  water  level,  and  en 
riched  sulphides  commonly  occur  at  or  just  below  this  horizon. 
While  not  a  uniform  relation,  the  water  level  commonly  follows 
the  topography. 

The  ground  water  may  be  considered  to  fill  a  complicated 
but  connected  system  of  porous  rock  masses,  fissures,  joints, 
and  other  open  spaces.  In  most  instances  the  water  level  reflects 
the  surface  in  a  modified  form;  being  deepest  below  the  middle 
slopes,  and  becoming  shallow  again  in  the  foothills,  where  the 
water  reaches  the  surface  in  springs.  In  the  Southwestern 
States,  the  valleys  are  commonly  filled  with  debris;  the  appar- 
ent foothills,  therefore,  are  often  the  middle  slopes  as  referred  to 
the  outlet  of  ground  water,  and  oxidation  may  proceed  to  im- 
11 


162  EXAMINATION  OF  PROSPECTS 

portant  depths  beneath  them.  The  ground-water  level  depends 
upon  the  permeability  of  the  rocks  traversed  as  well  as  upon  the 
height  of  outlet.  The  water  level  should  be  horizontal  beneath 
a  hill  of  loose  conglomerate,  for  example,  but  might  easily  reach 
the  surface  in  a  tight  rock  that  does  not  permit  seepage. 

In  investigating  the  probable  depth  of  the  water  level  in  a  new 
district,  it  should  be  borne  in  mind  that  springs  may  exist  at 
levels  considerably  above  the  general  ground-water  level,  being 
the  mouths  of  accidental  drainage  channels  that  permit  a  more 
easy  escape  of  waters  than  does  continued  seepage.  A  number 
of  springs  at  approximately  the  same  elevation  is  commonly  a 
sign  of  the  true  water  level. 

Where  there  has  been  much  faulting,  a  district  may  be  divided 
into  fault  blocks,  each  of  which  constitutes  a  separate  hydro- 
static basin,  the  water  being  impounded  to  a  ceratin  extent  by 
the  impervious  gouge  along  the  fault  planes.  The  ground- 
water  level  is  likely  to  be  deep  in  arid  regions,  and  shallow  in 
regions  of  heavy  rainfall,  and  the  same  relation  holds  for  the 
depths  to  which  oxidation  may  be  expected  to  penetrate. 

Where  the  water  level  is  permanent,  well-defined,  and  not  too 
deep,  oxidation  and  enrichment  tend  to  form  relatively  regular 
zones;  where  the  water  level  is  irregular  or  practically  lacking, 
oxidation  and  enrichment  are  likely  to  be  quite  irregular.  The 
water  level  in  any  district  is  likely  to  change  with  time,  and  the 
level  at  some  past  epoch  may  have  been  the  determining  factor 
in  the  location  of  enrichments  as  now  found. 

The  Irregularity  of  Oxidation  and  Enrichment. — A  large  mass 
of  primary  ore  uniformly  mineralized  and  brecciated,  or  of  uniform 
porosity,  should  yield  upon  oxidation  and  enrichment  bodies 
of  secondary  ores  of  uniform  depth  that  bear  a  like  definite 
relation  to  the  topography.  This  result,  rarely  attained,  finds  its 
nearest  expression  in  certain  disseminated  chalcocite  enrichments. 
y  Oxidation  and  enrichment  persist  deepest  along  the  more 
prominent  zones  or  lines  of  fracture,  and  it  is  not  unusual  to  find 
tongues  of  secondary  ores  deep  within  the  primary  zone.  Fur- 
thermore, homogeneous  masses  of  unaltered  primary  sulphides 


ALTERATIONS  BY  SURFACE  AGENCIES  163 

are  often  found  in  the  oxidized  zone,  having  been  protected  by 
their  massive  structure  against  attack  by  oxidizing  and  dissolv- 
ing solutions.  In  a  search  for  secondary  ores,  therefore,  the 
post-mineral  fractures  should  be  carefully  studied,  and  explora- 
tion should  be  directed  to  expose  their  intersections  with  the 
zones  of  primary  mineralization. 

It  occasionally  happens  that  the  role  played  by  fracturing  in 
the  formation  of  secondary  enrichments  is  emphasized  by  the 
occurrence  of  such  enrichments  along  post-mineral  fissures 
or  joint  planes,  the  precipitation  having  been  accomplished  by 
absorption,  or  other  agencies. 

The  Zones  Developed  by  Surface  Agencies. — The  ideal  result 
of  the  rearrangement  of  a  primary  ore-body  by  surface  waters 
would  be  the  formation  of  a  surface  zone  of  resistant  minerals, 
leached  of  their  metals,  beneath  which  would  be  found  an 
oxidized  zone  of  low  grade,  and  in  turn  at  successively  greater 
depths,  a  zone  of  rich  carbonate  and  oxide  ores,  and  a  zone  of 
secondary  sulphide  enrichment  resting  upon  the  primary  ores. 
The  complete  series  is  occasionally  developed,  but  most  deposits 
exhibit  irregularities  due  to  structural  features,  relative  rapidity 
of  erosion  as  compared  to  solution  and  concentration,  differing 
solubilities  of  the  minerals  present,  and  the  preeipitative  action 
of  associated  rocks  or  minerals. 

The  oxidized  and  leached  zones  are  usually  deep  in  arid 
regions,  and  an  important  proportion  of  the  valuable  ore  is 
likely  to  occur  in  the  oxidized  zone;  in  wet  climates,  the  leached 
and  oxidized  zones  are  commonly  slightly  developed,  and  the 
major  proportion  of  the  valuable  ore  is  likely  to  be  found  as 
sulphide  enrichments. 

In  most  cases  the  enriched  ores  are  not  the  result  of  the 
leaching  of  the  present  oxidized  zone,  but  are  derived  from  a 
great  vertical  extent  of  vein  or  deposit,  the  residual  minerals 
of  which  have  been  removed  by  erosion;  the  present  enrichments 
may,  therefore,  be  considered  to  represent  repeated  reconcen- 
t  rations. 

The  actual  depths  of  the  various  zones  vary  widely  in  different 


164 


EXAMINATION  OF  PROSPECTS 


districts,  and  in  different  parts  of  the  same  district,  or  even  in  the 
same  deposit,  depending  upon  the  local  conditions  of  water 
circulation.  The  depths  at  which  changes  commonly  occur  in 
other  districts  do  not  form  reliable  guides  in  the  examination 


Surface 


FIG.  71. — Diagram  showing  the  occurrence  of  oxidized,  enriched,  and 
primary  ore  in  the  Granite  vein,  Phillipsburg,  Montana;  the  horizontal 
dimensions  of  the  areas  show  approximately  the  relative  amounts  of  each 
type  of  ore  at  various  depths,  but  are  without  significance  as  to  their  posi- 
tions in  the  vein.  After  W.  H.  Emmons. 

of  a  new  deposit.  The  several  zones  occasionally  are  sharply 
marked,  and  the  change  comes  without  warning,  but  commonly 
the  zones  grade  gradually  one  into  the  next,  and  so  give  warning 
of  the  impending  change. 


ALTERATIONS  BY  SURFACE  AGENCIES  165 

While  it  is  not  possible  to  predict  the  depths  at  which  the 
several  zones  will  occur,  it  is  possible  and  of  great  practical 
importance  to  recognize  the  zone  exposed,  and  so  to  appreciate 
the  changes  that  may  reasonably  be  expected  with  deeper  ex- 
ploration. The  relative  importance  of  the  several  zones  depends 
in  large  measure  upon  the  minerals  and  metals  of  the  primary 
ore.  Copper,  for  example,  is  frequently  completely  leached  from 
the  surface  zone  and  is  found  as  chalcocite  enrichments  without 
overlying  residual  ores.  Where  oxidation  proceeds  more  rapidly 
than  solution  and  enrichment,  as  for  example,  in  gold  deposits, 
the  oxidized  ore  commonly  contains  the  bulk  of  the  values  of 
the  primary  ore,  perhaps  in  concentrated  form,  while  the  zone 
of  enriched  sulphides  is  poorly  developed,  or  lacking.  The 
upper  part  of  the  enriched  zone  is  commonly  the  richest  horizon 
of  a  secondary  deposit. 

In  general,  the  completeness  with  which  any  ore-body  is 
rearranged  by  oxidation  is  proportional  to  the  relative  solu- 
bilities of  the  metals  it  contains  in  the  presence  of  existing 
precipitants,  and  the  degree  of  post-mineral  fracturing,  or  of 
porosity  due  to  abundant  sulphides  as  compared  with  resistant 
gangue  minerals. 

In  the  upper  zones  of  deposits  that  have  been  subject  to 
oxidation  and  enrichment,  kernels,  or  residual  masses,  are 
likely  to  be  found  that  represent  the  character  of  the  zones  below. 
Residual  nodules  of  oxidized  ores  are  likely  to  occur  in  the 
leached  surface  zone,  nodules  of  enriched  sulphides  in  the  oxidized 
zone,  and  masses  of  residual  primary  ore  in  the  zone  of  sulphide 
enrichment.  A  study  of  such  residual  masses  may  yield  in- 
formation in  regard  to  the  character  of  the  lower  zones  not  yet 
exposed  by  exploration;  a  free  milling  oxidized  gold  ore,  for 
example,  may  contain  masses  of  unaltered  and  refractory 
pyritic  ore,  indicating  that  such  will  be  the  nature  of  the  primary 
ore  when  reached.  Residual  kernels  of  galena  frequently  carry 
higher  values  in  silver  than  does  this  mineral  in  the  primary  ore. 

AT  PHILLIPSBURG,  MONTANA,  the  Granite  Vein1  affords  a  good 

1  W.  H.  Emmons,   Butt.  315,  U.  S.  G.  S.,  p.  42. 


166  EXAMINATION  OF  PROSPECTS 

example  of  the  rearrangement  of  values  by  surface  agencies. 
The  vein  has  suffered  post-mineral  movement  and  much  of  the 
ore  is  fractured,  permitting  ready  access  to  percolating  surface 
waters.  The  primary  ore  consists  of  pyrite,  arsenopyrite, 
tetrahedrite,  and  tennantite,  with  some  galena  and  zincblende. 
Sparingly  scattered  through  this  ore  are  small  specks  of  pyaragy- 
rite,  proustite,  and  lesser  amounts  of  realgar  and  orpiment. 
The  primary  ore  carries  from  20  to  30  oz.  of  silver  and  from 
$1.50  to  $3.00  in  gold;  the  gangue  is  quartz  and  rhodochrosite. 
The  secondary  or  enriched  sulphide  ore,  resting  upon  the  primary 
ore,  consists  of  bands  of  quartz  and  rhodochrosite  alternating 
with  bands  of  rich  sulphides,  comprising  argentite,  proustite, 
pyrargyrite,  tetrahedrite,  tennantite,  pyrite  and  arsenopyrite. 
Galena  and  zincblende  are  locally  abundant;  chalcocite  and 
bornite  are  of  rare  occurrence.  This  ore  carries  from  50  to  1000 
oz.  of  silver  and  from  $4  to  $8  in  gold.  Probably  more  than  one- 
half  of  the  silver  in  this  ore  is  present  as  dark  ruby  silver,  or 
pyrargyrite,  with  some  proustite,  occurring  in  minute  veinlets 
or  seams  filling  cracks  in  the  vein,  as  films  on  the  outside  of 
crushed  vein  material,  or  as  relatively  large  crystals  lining 
cavities.  This  ore  is  responsible  for  a  large  proportion  of  the 
dividends  that  the  mine  has  paid.  The  enriched  oxidized  ore 
occurs  just  above  the  enriched  sulphide  ore;  it  is  composed  of 
quartz  stained  by  the  oxides  of  iron  and  manganese,  and  less 
commonly  by  copper  carbonates.  Cerargyrite  and  native  silver 
occur  as  thin  seams  cutting  through  the  quartz  and  as  films  on 
the  outside  of  quartz  fragments.  Associated  with  this  ore  are 
small  quantities  of  argentite  and  pyromorphite,  and  residual 
galena,  zincblende,  pyrite  and  chalcopyrite  are  of  local  occur- 
rence. This  ore  carries  from  300  to  400  oz.  of  silver  and  from 
$5  to  $16  in  gold.  The  poor,  or  leached,  oxidized  ore  forms  the 
upper  zone;  it  is  composed  of  quartz,  commonly  broken  and 
stained  by  iron  and  manganese  oxides.  It  carries  small  quantities 
of  lead  carbonate,  malachite,  azurite,  chrysocolla,  pyromor- 
phite, and  residual  pyrite  and  galena;  it  carries  less  than  40  oz. 
of  silver,  and  but  little  gold.  The  sulphides  contained  by  the 


ALTERATIONS  BY  SURFACE  AGENCIES  167 

oxidized  ores  are  remnants  that  have  escaped  oxidation, 
their  massive  structure  having  prevented  the  access  of  the 
oxidizing  solutions;  low-grade  pyrite  occurs  locally  as  druses  of 
secondary  origin. 

The  Depth  of  Vein  Leached  to  Form  Existing  Enrichments.— 
The  metallic  content  of  the  enriched  ores  in  any  deposit  is  rarely 
due  to  the  leaching  of  the  present  depth  of  the  oxidized  and 
leached  zones,  but  is  probably  derived  from  a  large  vertical 
extent  of  vein  now  removed  by  erosion.  It  is  occasionally 
possible  to  calculate  approximately  the  depth  worked  over  by 
surface  agencies  necessary  to  have  produced  the  existing 
enrichments. 

AT  PHILLIPSBURG,  MONTANA,  the  Granite  Vein1  yields  data  upon 
which  to  calculate  the  depth  of  primary  ore  worked  over  necess- 
ary to  account  for  the  enrichments  found.  Here  the  primary 
ore  carries  about  25  oz.  of  silver.  The  enriched  ores,  about  400 
ft.  in  vertical  extent  in  one  part  of  the  mine,  average  175  oz.  of 
silver.  On  the  basis  of  these  figures,  assuming  a  constant  width, 
the  depth  of  primary  ore  necessary  to  produce  the  enrichment, 
must  have  been  about  2400  ft.,  provided  that  all  of  the  contained 
silver  found  its  way  into  the  enriched  ores. 

1  W.  H.  Emmons,  Bull.  315.  U.  S.  G.  S.,  p.  43. 


CHAPTER  IX 
RESIDUAL  ORES  AND  THEIR  DISTRIBUTION 

The    Precipitation    of    Ores    in  The   Zone  of  Oxidation. — In 

deposits  in  which  the  secondary  zones  are  well-defined  a  layer 
of  rick' oxidized  ore  is  frequently  found  immediately  to  overlie 
the  enriched  sulphides,  from  which  it  is  derived  by  direct  oxida- 
tion in  place.  In  deposits  that  contain  oxidizing  percipitants 
particles  and  masses  of  residual  oxidized  ore  are  likely  to  be 
scattered  through  the  leached  zone,  having  been  precipitated 
during  migration  before  reaching  the  zone  of  enrichment.  This 
precipitation  hinders  secondary  enrichment  and  in  extreme 
cases  prevents  the  formation  of  such  concentrations. 

R&J^luil^  oros  <ire  precipitated  by  various  reagents.  Car- 
bonates are  formed  through  the  action  of  the  carbonic  acid  con- 
tained in  surface  waters,  and  also  directly  through  the  replace- 
ment of  calcite,  especially  where  the  containing  rock  is  limestone. 
Native  metals,  such  as  copper,  gold,  and  silver,  are  formed  by 
the  action  of  reducing  agents,  among  which  organic  matter  and 
ferrous  sulphate  are  prominent.  Kaolin,  gouge,  and  certain 
shales  occasionally  act  as  powerful  precipitants  through  adsorp- 
tion. Silver  is  often  found  as  residual  chloride,  formed  through 
precipitation  by  the  chlorine  contained  in  surface  waters. 

Residual  ores  are  often  the  result  of  incomplete  solution,  the 
relatively  insoluble  minerals  being  left  behind  during  the  migra- 
tion of  associated  metals;  incomplete  oxidation  and  solution 
often  leave  residual  masses  of  unaltered,  or  partly  altered,  sul- 
phides in  the  oxidized  zone.  Such  residual  particles  or  masses 
have  commonly  been  enriched  by  additions  from  circulating, 
metal-bearing  solutions.  ^ 

During  oxidation  under  conditions  that  permit  reprecipitation, 
such  as  the  oxidation  of  sulphide  deposits  in  limestone,  a  scat- 

168 


RESIDUAL  ORES  AND  THEIR  DISTRIBUTION          169 

tered  primary  mineralization  is  probably  often  segregated  into 
masses  of  rich  oxidized  minerals  without  important  vertical 
migration.  In  the  zinc  and  lead  deposits  of  the  Mississippi 
Valley  the  lead  is  usually  found  as  residual  enrichments  of 
galena,  above  the  water  level,  while  the  zinc  and  iron  sulphides 
have  in  great  part  been  removed  by  solution  and  redeposited 
at  and  below  the  water  level. 

Oxidized  Ores  of  Copper.  —  Native  copper  is  the  last  stage  in 
the  reduction  of  copper  compounds,  and  is  frequently  found  as 
pellets  or  films  in  the  upper  oxidized  zones  of  copper  deposits. 
It  is  commonly  associated  with  cuprite,  from  which  it  is  probably 
derived  in  most  cases.  Cuprite  is  often  found  just  above  the 
chalcocite  zone,  where  it  is  formed  from  the  chalcocite  by  direct 
oxidation.  Both  native  copper  and  cuprite  are  commonly 
indicative  of  long  and  thorough  oxidation,  and  are  frequently 
found  above  important  chalcocite  enrichments.  Tenorite,  or 
melaconite,  is  intimately  related  to  and  associated  with  chalco- 
cite. Chrysocolla  is  a  common  residual  ore  of  copper,  and  is  more 
likely  to  be  abundant  in  siliceous  rocks  than  in  limestone,  where 
carbonates  are  likely  to  prevail.  The  silicate  of  copper  is  fre- 
quently associated  with  manganese  in  black  compounds  of 
indefinite  composition  commonly  called  copper-pitch  ores. 
Malachite  and  azurite  are  among  the  most  important  oxidized 
copper  minerals;  they  are  relatively  resistant  to  oxidizing  pro- 
cesses and  are  frequently  found  as  residual  ores.  Azurite  appears 
to  be  the  more  resistant  of  the  two.  Malachite  and  azurite 
frequently  occur  with  limonite,  an  association  that  is  explained 
on  the  supposition  that  siderite  was  formed  with  the  copper 
carbonates,  but  subsequently,  being  more  susceptible  to  altera- 
tion, decomposed  to  limonite.  Brochantite  is  occasionally 
an  important  oxidized  ore  of  copper,  being  often  a  transitional 
step  in  the  formation  of  carbonates.  In  desert  climates,  ataca- 
mite  may  constitute  an  important  ore,  though  its  easy  solu- 
bility confines  it  to  regions  of  extreme  aridity. 

Oxidized  Ores  of  Lead. — The  only  important  primary  ore  of 
lead  is  galena,  which  is  relatively  resistant  to  oxidation,  and  is 


170  EXAMINATION  OF  PROSPECTS 

frequently  found  near  the  surface.  Upon  oxidation  it  alters 
slowly  to  sulphate  and  carbonate,  relatively  insoluble  products 
that  commonly  do  not  migrate  far  before  being  precipitated. 
Under  the  attack  of  sulphate  solutions  galena  alters  to  anglesite, 
which  may  remain  as  a  residual  ore;  more  frequently  the  angle- 
site  is  altered  to  cerussite,  which  is  the  less  soluble  compound. 
In  certain  districts,  as  for  example,  the  Coeur  d'Alenes1  where 
galena  is  in  primary  association  with  siderite,  the  latter  mineral 
upon  alteration  to  limonite  sets  free  abundant  carbonic  anhydride, 
and  the  galena  passes  into  cerussite  apparently  without  the 
formation  of  anglesite. 

Oxidation  occasionally  proceeds  to  great  depths  in  lead 
deposits,  and  the  bulk  of  the  ore  is  found  as  residual  particles  or 
masses  that  have  been  added  to  somewhat  by  enrichment  through 
solution  and  precipitation.  Pyromorphite,  and  the  oxides  of 
lead,  plattnerite  and  massicot,  are  of  less  common  occurrence, 
and  are  usually  characteristic  of  the  upper  parts  of  the  oxidized 
zone,  in  which  lead  chromate  and  molybdate  also  are  occasion- 
ally found. 

Where  residual  masses  of  galena  occur  in  the  oxidized  zone 
they  frequently  carry  silver  due  to  enrichment  by  migrating 
solutions,  and  are  higher  in  grade  than  the  primary  galena 
unaffected  by  surface  processes. 

Oxidized  Ores  of  Zinc. — Sphalerite,  or  zincblende,  the  only 
important  primary  zinc  mineral,  yields  readily  to  oxidation  with 
the  formation  of  zinc  sulphate,  a  very  soluble  salt  that  commonly 
migrates  far  before  being  precipitated. 

It  is  rare  to  find  any  traces  of  zinc  in  the  upper  parts  of  oxi- 
dized ore  bodies;  lower  down  in  the  deposits,  zinc  is  frequently 
precipitated  as  calamine  or  smithsonite  replacing  limestone  or 
other  sedimentary  rocks.  These  minerals  are  difficult  to  iden- 
tify, as  they  usually  reproduce  with  great  fidelity  the  structure 
of  the  replaced  rock,  and  their  low  specific  gravities  fail  to  call 
attention  to  their  high  metallic  content. 

1  F.  L.  Ransome,  P.  P.  62,  U.  S.  G.  S.,  p.  132. 


RESIDUAL  ORES  AND  THEIR  DISTRIBUTION  171 

AT  THE  HORN  SILVER  MINE,  UTAH/  a  secondary  distribution  of 
metals  has  taken  place  under  conditions  of  partial  oxidation. 
The  upper  400  ft.  of  the  ore-body,  whose  gangue  is  chiefly  quartz 
and  barite,  carries  lead-silver  ores,  the  lead  as  galena,  cerussite, 
anglesite  and  oxides,  and  the  silver  as  cerargyrite  and  as  ruby 
silver;  zinc  and  copper  are  scanty  or  lacking.  At  700  ft.,  large 
bodies  of  zinc  ores  occur  as  carbonate  and  silicate  associated 
with  some  lead.  From  650  to  750  ft.  copper  comes  in,  occurring 
as  chalcocite,  with  which  is  associated  galena. 

Residual  Shoots  of  Gold  Ores. — The  .upper  parts  of  gold  de- 
posits are  commonly  richer  than  the  underlying  primary  ores. 
This  residual  enrichment  in  the  zone  of  oxidation  is  in  large 
degree  owing  to  the  solution  and  migration  of  associated  min- 
erals, the  reduction  in  mass  giving  rise  to  a  concentration  of  the 
undissolved  gold.  A  further  factor  in  this  concentration  is  the 
actual  solution,  migration  and  enrichment  of  gold. 

The  solution  and  migration  of  gold  takes  place  more  slowly 
in  most  cases  than  the  migration  of  associated  metals;  this 
results  in  the  zone  of  gold  enrichment  being  left  at  a  horizon 
above  the  enrichments  of  associated  metals.  The  minerals 
that  commonly  occur  with  gold  in  the  upper  parts  of  oxidized 
ore-bodies  are  quartz  and  limonite ;  the  limonite  is  often  removed 
by  solution,  leaving  the  gold  in  a  porous  mass  of  quartz,  the 
result  being  a  still  further  concentration  of  the  gold  with  respect 
to  the  containing  gangue. 

A  surface  enrichment  frequently  takes  place  in  gold  deposits 
during  the  disintegration  of  the  outcrop  on  weathering;  the 
lighter  grains  of  gangue  are  washed  or  blown  away  and  the 
gold  particles  tend  to  work  downward  into  superficial  cracks, 
forming  enrichments  that  usually  extend  for  very  short  distances 
only  below  the  surface.  Where  erosion  or  glaciation  proceeds 
more  rapidly  than  oxidation,  the  soft  upper  parts  of  ore-deposits 
are  removed  and  primary  ores  outcrop  at  the  surface. 

Shoots  of  residual  gold  ores  must  be  considered  as  surface 
phenomena.  Their  greatest  dimension  is  more  likely  to  be 

1S.  F.  Emmons,  Trans.,  A.  I.  M.  E.,  XXXI,  p.  658. 


172  EXAMINATION  OF  PROSPECTS 

horizontal  than  vertical,  being  confined  to  the  zone  above  the 
water  level,  or  the  depth  to  which  oxidation  has  penetrated. 

The  Distribution  of  Silver  by  Oxidation. — Silver  ores  are  af- 
fected in  various  ways.,  by  oxidation.  The  sulphate  of  silver, 
being  readily  soluble,  migrates  freely,  and  may  precipitate  as 
sulphide  enrichments  at  the  water  level.  Precipitation  of  silver 
as  chloride  is  frequent,  however,  in  the  zone  of  oxidation,  especi- 
ally in  arid  regions  where  the  residual  ores  are  likely  to  be  the 
most  important:  finally,  silver  chloride  is  slightly  soluble,  and, 
migrating  through  relatively  short  distances,  is  likely  to  produce 
local  enrichments  in  the  oxidized  zone.  A  residual  concentra- 
tion of  silver  as  chloride  and  native  silver  may  also  take  place 
through  the  removal  by  solution  of  relatively  soluble  gangue  min- 
erals. Minerals  that  are  frequently  associated  with  cerargyrite  in 
oxidized  ores  are  native  silver,  the  bromide  and  the  iodide. 

The  residual  ores  of  silver,  being  confined  to  the  zone  of  oxida- 
tion, commonly  show  a  distribution  related  to  the  topography. 

The  Distribution  of  Manganese  by  Oxidation. — The  oxides  of 
manganese  are  frequently  found  as  residual  concentrations  in 
the  oxidized  zone.  Indeed,  it  is  sometimes  difficult  to  explain 
their  abundance  in  the  oxidized  parts  of  certain  deposits  whose 
primary  ore  contains  but  little  of  this  element.  The  oxides  of 
iron  and  of  manganese  frequently  occur  with  residual  concentra- 
tions of  both  silver  and  of  gold.  Manganese  oxide  usually  is 
most  abundant  in  the  upper  part  of  the  oxidized  zone. 

The  relative  abundance  of  manganese  in  the  oxidized  zone 
as  compared  with  the  primary  zone  often  gives  an  idea  in 
regard  to  depth  of  leaching  and  erosion  necessary  to  produce 
such  concentrations. 


CHAPTER  X 
SECONDARY  ORES  AND  ORE-SHOOTS 

The  Criteria  of  Secondary  Ores. — Secondary  minerals  are  the 
result  of  a  process  of  concentration  and  enrichment  and  are 
commonly  richer  then  the  primary  minerals  of  the  same  deposit. 
Secondary  ores  that  contain  abundant  sulphides  are  commonly 
distinguishable  in  the  field  from  primary  ores;  this  distinction 
is  more  difficult  with  ores  that  carry  a  small  quantity  only  of 
sulphide  minerals,  and  microscopical  examination  of  the  ore  in 
thin  section  is  advisable  in  all  doubtful  cases. 

It  is  frequently  difficult,  and  in  some  cases  impossible,  to 
determine  the  primary  or  secondary  character  of  quartzose  gold 
ores.  The  safest  guide  is  the  presence  or  absence  of  gaseous  or 
fluid  inclusions  in  association  with  the  ore  minerals;  such  primary 
association  coupled  with  a  lack  of  traces  of  oxidation  is  indicative 
of  primary  origin. 

Any  ore  that  carries  traces  of  oxidation,  such  as  iron  stains, 
dendritic  oxides  of  manganese,  kaolin  and  so  forth  must  be  con- 
sidered to  have  been  acted  upon  by  surface  agencies,  and  secondary 
enrichment  must  be  suspected.  Further  examination,  however, 
may  indicate  that  the  contained  values  are  primary,  and,  there- 
fore, persistent  in  their  vertical  distribution. 

The  manner  of  occurrence  of  secondary  minerals  is  frequently 
characteristic;  secondary  minerals,  being  due  to  the  action  of 
surface  waters,  are  commonly  found  in  cracks  through  earlier 
minerals,  or  as  crusts  lining  vugs  or  surrounding  nucleal  masses, 
or  in  general,  in  some  distribution  later  than  and  not  conform- 
ing to  the  arrangement  of  the  primary  minerals.  Secondary 
sulphides  are  often  found  as  soft  black  pulverulent  masses  or 
coatings,  although  they  frequently  occur  well-crystallized  or  as 
compact  masses  of  metallic  luster.  Where  the  relationship 

173 


174 


EXAMINATION  OF  PROSPECTS 


FIG.  72. — Photomicrograph  of  secondarily  enriched  ore  from  the  lower 
part  of  the  chalcocite  zone,  Morenci,  Arizona,  showing  the  occurrence  of  the 
later  and  richer  sulphide  through  the  pyrite.  Dark  gray  =  secondary 
chalcocite,  developing  by  replacement  in  pyrite;  light  gray  =  pyrite;  black 
=  open  field.  Ajler  Lindgren. 


SECONDARY  ORES  AND  ORE-SHOOTS 


175 


may  be  determined,  secondary  ores  are  always  found  in  connec- 
tion with  post-mineral  fractures,  or  beneath  a  porous  capping. 
The  form  of  an  ore-deposit  often  indicates  its  secondary 
origin.  This  criterion,  however,  is  not  available  until  after  the 
deposit  has  been  partly  explored.  An  ore-deposit  formed  above 
and  resting  on  an  impervious  stratum  is  probably  the  result  of 
descending  waters  and  is  of  secondary  origin.  A  deposit  that 


Chalcocite 


FIG.  73. — Group  of  pyrite  crystals,  showing  secondary  chalcocite  along  the 

After  Paige. 


branches  as  it  is  followed  downward,  leaving  the  channel  of 
primary  mineralization  to  become  enclosed  in  the  wall  rocks,  is 
certainly  of  secondary  origin. 

Most  veins  are  lines  of  weakness  likely  to  be  reopened  by 
post-mineral  fractures ;  surface  waters  frequently  find  their  way 
to  great  depths  along  such  fractures  and  secondary  processes 
may  be  active  to  any  depth  to  which  they  penetrate.  The 
range  of  influence  of  these  processes  may  be  appreciated  upon 
considering  the  number  of  deep  mines  that  are  wet. 

A  list  of  the  minerals  that  are  formed  by  oxidizing  processes  is 
given  in  a  preceding  chapter.  Many  minerals  that  are  found  as 


176  EXAMINATION  OF  PROSPECTS 

secondary  enrichments  are  also  formed  by  primary  processes, 
and  their  occurrence,  therefore,  may  not  be  accepted  as  proof  of 
secondary  origin.  Minerals  that  are  secondary  in  a  great  majority 
of  their  known  occurrences  are  covellite,  chalcocite,  kaolin,  chal- 
cedony, and  the  sulpharsenites  and  sulphantimonites. 

MINERALS  FORMED  IN  LOWER  GROUND-WATER  ZONE 
(Zone  of  Secondary  Sulphide  Enrichment1) 

Quafetz  Bornite 

Chalcedony  Covellite 

Opal  Chalcocite 

Kaolin  Argentite 

Gold  Pyrargyrite 

Silver  Stephanite 

Pyrite  Polybasite 

Galena  Other    complex    sulpharsenites 

Chalcopyrite  and  sulphantimonites. 

Secondary  Ore -Shoots. — Secondary  ore-shoots  are  the  results  of 
surface  agencies  and  bear  a  definite  relation  to  the  surface,  and 
in  any  district  they  are  likely  to  be  found  within  certain  limits 
of  depth.  In  shallow  deposits,  the  vertical  distribution  fre- 
quently reflects  the  topography,  while  deep  enrichments  com- 
monly occur  in  more  nearly  horizontal  zones.  The  greatest 
dimension  of  secondary  ore-shoots  is,  in  general,  horizontal,  while 
in  primary  ore-shoots  the  reverse  is  the  rule.  Furthermore,  in 
secondary  deposits  the  change  in  values  is  likely  to  be  most 
pronounced  vertically,  while  in  primary  deposits  the  best 
defined  changes  in  value  are  commonly  in  the  direction  of  their 
strike. 

Secondary  processes  affect  in  like  manner  all  types  of  primary 
deposits,  the  degree  of  alteration  and  reconcentration  depending 
upon  the  original  constituents  of  the  deposits  and  upon  the 
structural  features  that  control  the  passage  of  waters.  In  the 

1  Waldemar  Lindgren,  Economic.  Geology,  II,  p.  124. 


SECONDARY  ORES  AND  ORE-SHOOTS  177 

consideration  of  secondary  ore-shoots  the  primary  distribution 
of  values  must  not  be  lost  sight  of;  secondary  enrichments 
commonly  occur  at  certain  horizons  within  the  primary  ore- 
shoots,  although  cases  are  not  rare  where  secondary  enrichment 
has  affected  veins  for  long  distances  along  their  strike,  the 
primary  mineralization  of  which  was  probably  irregular. 

In  exploration  for  flat-lying  or  bed-like  enrichments,  such  as  the 
common  type  of  disseminated  chalcocite  enrichments,  the 
workings  should  be  vertical,  so  as  to  pass  through  the  successive 
zones.  In  exploration  for  possible  tongues  of  secondary  ores 

s  w 


Surface. 


ren  and  Leached  Zone 


4900 . 

Chalcolite  Zone 


4800_ 


47QO  Zone  of  Primary  Cupriferous  Pyrite 


and  ZLnc  Blende 

46Qo____ 


FIG.  74.  —  Longitudinal  section  of  a  part  of  the  Humboldt  lode,  Morenei, 
Arizona,  showing  the  relations  of  the  leached,  enriched,  and  primary  zones. 
After  Lindgren. 

along  post-mineral  fractures  in  the  zone  of  primary  mineraliza- 
tion, however,  the  workings  should  be  horizontal;  vertical 
exploration  within  the  zone  of  primary  ores  is  futile. 

In  the  investigation  of  a  secondary  ore-deposit  it  is  wise  to 
average  all  samples  taken  from  each  level;  a  comparison  made 
between  the  different  levels  will  often  indicate  the  trend  of  in- 
crease or  decrease  in  values  as  depth  is  attained,  and  so  perhaps 
afford  a  basis  upon  which  to  form  an  opinion  of  the  continuity 
of  the  ore-shoots  in  depth. 

AT  MORENCI  AND  METCALF,  ARIZONA,  the  important  chalcocite 
enrichments  all  occur  high  up  on  the  hills;  erosion  appears  to 
have  proceeded  faster  than  leaching  and  enrichment,  and  the 
12 


178  EXAMINATION  OF  PROSPECTS 

canyon  bottoms  are  all  in  rock  carrying  a  primary  pyritic  minerali- 
zation only.  The  elevation  of  the  surface  is  here  a  guide  in 
exploration;  the  higher  slopes  are  considered  promising  ground, 
while  explorations  from  the  canyon  bottoms  cannot  be  expected 
to  yield  results. 

The  Effect  of  Structural  Feature  on  Secondary  Ore -Shoots.— 
Post-mineral  fractures  are  the  best  guides  in  a  search  for  second- 
ary ore-deposits;  these  deposits  are  in  every  case  due  to  surface 
waters,  which  circulate  most  abundantly  along  fractures,  faults, 
brecciated  zones,  joint  planes,  or  other  openings.  Any  search 
for  secondary  ore-bodies,  therefore,  should  be  directed  to  explore 
the  intersections  of  post-mineral  fractures  with  the  general 
trend  of  the  primary  mineralization.  Solid,  impervious  masses 
of  primary  ore  are  not  subject  to  enrichment,  and  commonly  carry 
their  primary  mineralization  unaltered  close  to  the  surface. 
Thorough  reopening  of  fractures  during  oxidation  constitutes 
the  most  favorable  condition  for  the  formation  of  secondary 
enrichments.  The  general  association  of  secondary  ores  with 
fractured,  shattered,  or  permeable  rocks  cannot  be  too  strongly 
emphasized. 

The  Effect  of  the  Water  Level  on  Secondary  Ore-Shoots. — In 
evenly  shattered  or  uniformly  permeable  deposits  precipitation 
and  enrichment  generally  take  place  at  the  water  level,  where 
chemical  conditions  change  and  precipitants  in  the  form^  of 
primary  sulphides  are  likely  to  be  met.  Where,  however,  a 
prominent  fracture  cuts  a  primary  ore-deposit,  and  forms  an 
important  channel  for  descending  solutions,  secondary  enrich- 
ments are  likely  to  persist  deep  into  the  zone  of  primary  ores. 
The  most  favorable  conditions  for  large  secondary  enrichments 
are  a  repeated  reopening  of  the  fractures  coupled  with  a  gradual 
sinking  of  the  water  level. 

The  Effect  of  Chemically  Active  Wall  Rocks  on  Secondary  Ore- 
Shoots. — In  deposits  that  contain  active  precipitants,  or  that 
are  contained  within  rocks  having  like  effect,  migration  and 
enrichment  do  not  take  place,  the  metals  being  precipitated 
immediately  upon  going  into  solution.  Where  a  deposit  traverses 


SECONDARY  ORES  AND  ORE-SHOOTS  .      179 

different  rocks  one  of  them  may  permit  migration  and  enrich- 
ment, while  the  others  may  impede  or  stop  these  processes. 

Acid  igneous  rocks,  and  acid  gangue  minerals  appear  to  be 
favorable  to  migration  and  enrichment,  while  sedimentary  rocks 
and  carbonate  gangue  minerals  appear  to  be  unfavorable.  Con- 
tact deposits,  on  account  of  their  resistant  minerals  and  their 
commonly  strong  precipitative  action,  rarely  contain  secondary 
enrichments,  although  such  enrichments  are  not  rare  in  associated 
intrusives. 

AT  CLIFTON,  ARIZONA.  1 — The  veins  where  contained  in  porphyry 
carry  important  secondary  chalcocite  enrichments,  but  where 
they  enter  limestone  or  shale,  they  become  impoverished. 

The  Effect  of  Porosity  on  Secondary  Ore -Shoots. — Most  fresh 
rocks  are  massive  and  impervious,  except  certain  sediments, 
whose  active  precipitative  action  prevents  migration  and  en- 
richment. Alteration,  usually  involving  kaolinization,  is  there- 
fore a  usual  prerequisite  of  enrichment.  The  ease  with  which 
rocks  or  gangue  minerals  alter  to  a  permeable  mass  is  an  important 
factor  in  secondary  enrichment,  aside  from  the  existence  of  the 
necessary  post-mineral  fracturing  that  gives  access  to  the  surface 
waters. 

The  Effect  of  Primary  Mineralization  on  Secondary  Ore- 
Shoots. — Disseminated  enrichments  are  commonly  confined  to 
areas  whose  outcrops  bear  traces  of  abundant  primary  minerali- 
zation. The  tendency  in  vein-like  deposits,  however,  is  toward 
the  confinement  of  descending  and  enriching  waters  within  the 
vein,  and  thus  produce  relatively  long  enrichments  whose  dis- 
tribution is  not  so  definitely  controlled  by  the  primary  ore-shoots. 

Vein  deposits  are  commonly  more  persistent  vertically  than 
brecciated  zones,  and  enrichments  in  veins  may  be  due  to  the 
leaching  of  many  thousands  of  feet  of  the  vein  now  removed  by 
erosion,  resulting  perhaps  in  a  great  concentration,  and  a  wide 
difference  in  value  between  the  primary  and  the  secondary  ores. 
In  disseminated  deposits  there  is  usually  less  difference  in  value 
between  the  primary  and  the  secondary  ores. 

1  Waldemar  Lindgren,  P.  P.  43,  U.  S.  G.  S.,  p.  204. 


180  EXAMINATION  OF  PROSPECTS 

Ores  Containing  Both  Sulphide  and  Oxidized  Minerals. — In  arid 
regions  where  the  water  level  is  deep  and  the  supply  of  surface 
water  is  irregular,  a  zone  is  frequently  developed  in  which  the 
valuable  metals  occur  in  both  sulphide  and  oxidized  form,  with, 
in  general,  but  slight  enrichment  only  of  the  primary  values. 
The  occurrence  of  such  ores  through  a  considerable  vertical 
range  indicates  incomplete  solution  and  enrichment,  and  in  most 
cases,  the  absence  of  any  well-defined  zone  of  secondary  sulphides 
at  greater  depth. 

The  Enrichment  of  Copper. — Primary  copper  ores  commonly 
contain  abundant  pyrite,  and  are  actively  attacked  by  oxidation; 
the  sulphates  thus  formed  are  very  soluble,  but  yield  their 
copper  readily  as  secondary  sulphides  upon  coming  in  contact 
with  fresh  pyrite  at  the  water  -level.  As  a  result  of  these  con- 
ditions, copper  is  peculiarly  susceptible  to  solution  and  enrich- 
ment, and  the  best  defined  types  of  secondary  deposits  are  those 
of  copper. 

Chalcopyrite  is  the  usual  starting-point,  being  the  most 
abundant  primary  copper  mineral,  and  is  the  form  in  which  the 
copper  of  cupriferous  pyrite  is  supposed  to  exist.  Bornite, 
enargite,  tetrahedrite,  tennantite  and  occasionally  chalcocite  are 
primary  copper  minerals  of  lesser  importance.  The  secondary 
copper  sulphides  are  chalcocite,  chalcopyrite,  enargite,  bornite 
and  covellite,  the  last  named  being  probably  exclusively 
secondary  in  origin. 

On  account  of  the  strong  precipitative  action  of  calcite, 
chalcocite  enrichments  are  more  rare  in  limestone  than  in  other 
rocks,  except  where  the  primary  sulphides  occur  in  large  masses. 

Chalcocite  Enrichments. — The  final  stage  in  copper  enrich- 
ment is  chalcocite,  which  is  also  economically  the  most  important 
copper  mineral.  Kaolin  and  quartz  are  the  typical  associates  of 
secondary  chalcocite,  which  commonly  also  encloses  residual 
pyrite.  The  upper  part  of  the  chalcocite  zone  is  commonly  the 
richest. 

The  zones  developed  in  massive  pyritic  deposits  are  usually 
well-defined,  and  the  changes  from  oxidized  ores  to  chalcocite, 


SECONDARY  ORES  AND  ORE-SHOOTS  181 

and  from  chalcocite  to  pyrite  are  usually  sudden;  the  enrich- 
ment is  often  a  shallow  zone  of  nearly  pure  chalcocite.  Pyritic 
masses  that  have  been  shattered  commonly  carry  their  secondary 
chalcocite  in  seamlets,  or  as  coatings  on  nucleal  masses,  and  en- 
richment in  this  form  may  persist  through  considerable  depths. 

AT  BISBEE,  ARIZONA/  the  primary  mineralization  of  lean 
cupriferous  pyrite  occurs  in  large  bodies  in  limestone;  the  country 
rock  is  commonly  impregnated  with  pyrite,  and  minute  particles 
of  contact  minerals  in  the  vicinity  of  the  ore-bodies,  and  also 
in  places  exhibits  a  partial  silicification.  Secondary  processes 
have  transformed  and  enriched  these  primary  deposits,  resulting 
in  the  formation  of  ore-bodies  of  great  economic  importance.  In 
typical  occurrences  a  core  of  lean  cupriferous  pyrite  is  surrounded 
by  a  shell  of  pyrite  carrying  secondary  chalcocite,  commonly  as 
seamlets  and  coatings  on  the  pyrite  grains;  this  shell  is  surrounded 
in  turn  by  an  envelope  of  ferruginous  clays  containing  oxidized 
copper  ores.  Where  prominent  fractures  cut  the  pyritic  masses 
they  are  accompanied  by  chalcocite,  and  where  several  such 
fractures  occur  close  together  large  and  rich  ore-bodies  result.  In 
exploration,  when  a  drift  through  one  of  these  pyritic  masses 
shows  a  gradually  rising  copper  content,  it  is  known  that  the 
periphery  of  the  deposit  is  being  approached. 

AT  DUCKTOWN,  TENNESSEE,2  the  outcrops  consist  of  hydrous 
iron  oxide  associated  with  kaolin  and  quartz.  This  material, 
which  is  a  valuable  iron  ore,  extends  to  a  maximum  depth  of 
about  100  ft.  Below  this  iron  ore  there  is  commonly  a  few 
feet  of  chalcocite  ore,  which  in  most  of  the  deposits  lies  like  a 
floor  between  the  gossan  and  the  underlying  primary  sulphides; 
the  primary  ore  consists  of  pyrrhotite,  pyrite  and  chalcopyrite 
associated  with  lesser  amounts  of  zincblende,  bornite,  specularite 
and  magnetite,  with  a  gangue  of  lime  silicates,  quartz  and 
marmorized  limestone.  The  enclosing  country  rock  is  schist. 

AT  CLIFTON,  ARIZONA,  the  Copper  King  vein  exhibits  well  the 
several  zones  produced  by  surface  agencies.  The  outcrop  of 

1  F.  L.  Ransome,  P.  P.  21,  U.  S.  G.  S.,  p.  155. 

2  W.  H.  Emmons  and  F.  B.  Laney,  Bull.  470,  U.  S.  G.  S.,  p.  169. 


182  EXAMINATION  OF  PROSPECTS 

this  vein  is  white  quartz,  showing  in  a  few  places  casts  of  origin- 
ally contained  sulphides,  but  for  the  greater  part,  the  quartz  is 
massive,  and  unstained  by  iron.  This  outcrop  carried,  where 
underlain  by  the  ore-body,  a  little  malachite  and  chrysocolla  as 
stains  on  post-mineral  fractures  through  the  quartz.  Beneath 
the  hard  capping,  kaolin  is  mixed  with  the  quartz,  which  carries 
copper  stains  and  small  patches  of  residual  carbonates  and 
silicate  of  copper,  which  gradually  increase  in  quantity  until 
the  vein  becomes  ore.  Beneath  this  oxidized  ore,  chalcocite, 
associated  with  kaolin,  quartz,  and  cuprite,  comes  in,  gradually 
changing  to  chalcocite,  associated  with  pyrite  and  quartz,  which 
form  a  very  hard  and  tough  ore  for  several  hundred  feet  in  depth. 
The  pyrite  in  this  ore  becomes  more  prominent  as  depth  is  at- 
tained, until  upon  the  disappearance  of  the  chalcocite,  the  chief 
values  in  the  pyritic  material  are  as  chalcopyrite,  which  in  turn 
becomes  less  in  depth,  and  in  the  lower  part  of  the  deposit  the 
primary  ore  is  exposed,  consisting  of  pyrite,  associated  with  a 
little  chalcopyrite  and  zincblende  in  a  gangue  of  quartz  and 
quartz-porphyry.  The  pyritic  ores  carry  low  values  in  gold. 

Disseminated  Chalcocite  Enrichments. — Disseminated  chalco- 
cite enrichments,  popularly  called  "porphyry  copper  deposits/7 
now  form  one  of  the  principal  reserves  of  known  copper  ores; 
many  millions  of  tons  of  such  ores  averaging  perhaps  2  per  cent, 
copper  have  been  developed  at  different  camps  in  the  South- 
western States. 

The  primary  ore  of  this  type  of  deposit  is  disseminated  cuprif- 
erous pyrite,  usually  associated  with  quartz  and  accompanied 
by  sericitization  of  the  containing  rock.  This  mineralization  is 
commonly  introduced  into  crushed  or  sheared  zones,  the  minerals 
forming  veinlets  through  the  rocks;  it  also  frequently  follows 
joint  planes,  and  occurs  as  scattered  particles  through  the  rock 
between  such  fractures. 

Upon  the  oxidation  of  the  pyrite,  solutions  containing  the 
sulphates  of  iron  and  of  copper  and  sulphuric  acid  seep  downward 
through  the  mineralized  mass,  kaolinizing  the  rock  and  leaching 
it  of  its  contained  copper,  which  is  precipitated  on  coming  in 


SECONDARY  ORES  AND  ORE-SHOOTS  183 

contact  with  unaltered  pyrite  at  the  water  level.  This  process, 
while  very  irregular  over  short  distances,  produces  enriched 
deposits  that  are  relatively  uniform  when  considered  in  large 
masses.  The  enrichments  commonly  bear  a  definite  relation  to 
the  depth  below  the  surface  and  to  the  ground-water  level. 

In  some  instances  a  part  of  the  iron  is  left  behind  in  the  leached 
zone  as  limonite,  while  in  others  the  iron  is  completely  removed 
with  the  copper,  the  leached  capping  being  a  porous  mass  of 
white  kaolin  and  quartz.  In  the  upper  oxidized  zone,  where 
not  completely  leached,  large  bodies  of  low-grade  residual  ores 
are  frequently  found,  in  which  the  copper  occurs  as  chrysocolla, 
carbonate,  or  partly  oxidized  particles  of  sulphide  minerals,  and 
also  in  some  cases,  as  native  copper  and  cuprite.  The  low  grade 
of  these  residual  ores  coupled  with  the  presence  of  perhaps  an 
important  proportion  of  the  contained  copper  as  sulphate  results 
in  serious  metallurgical  difficulties,  due  to  solution  and  to  the 
low  specific  gravity  of  some  of  the  copper  compounds. 

Secondary  chalcocite  enrichments,  while  varying  greatly 
in  their  geological  associations,  present  a  remarkable  similarity 
to  casual  observation,  the  secondary  processes  having  tended 
to  produce  like  results  from  dissimilar  primary  deposits. 
Deposits  of  disseminated  copper  minerals  are  in  many  cases 
associated  with  monzonitic  intrusives. 

Important  features  in  the  consideration  of  these  deposits  are 
the  depth  at  which  they  occur,  whether  susceptible  to  stripping 
and  steam-shovelling,  and  the  strength  of  their  walls,  which  are 
commonly  kaolinized,  soft,  and  likely  to  cave  upon  the  removal 
of  any  large  quantity  of  ore. 

Inasmuch  as  these  deposits  are  commonly  of  horizontal  tabular 
form,  exploration  should  be  carried  out  vertically.  The  zone 
of  enrichment  is  expected  beneath  a  thoroughly  leached  and 
kaolinized  capping,  and  is  underlain  by  primary  pyritic  ores, 
into  which  it  is  futile  further  to  continue  vertical  exploration. 

The  primary  ore  may  usually  be  recognized  by  the  freshness  of 
the  containing  rock,  as  well  as  by  the  typical  primary  minerals. 
In  certain  oases  disseminated  enrichments  have  been  found 


184  EXAMINATION  OF  PROSPECTS 

beneath  pyritic  material,  but  always  where  this  pyritic  material 
was  disintegrated  and  the  associated  or  containing  rock  well 
kaolinized,  the  pyrite  having  been  leached  of  its  copper  without 
complete  oxidation  of  its  sulphur. 

AT  THE  INSPIRATION  MINE,  NEAR  MIAMI,  ARIZONA,  the  primary 
mineralization  occurs  as  an  impregnation  of  schists.  The  average 
depth  of  the  leached  capping1  is  367  ft.  The  average  thickness 
of  the  enriched  ore  is  155.5  ft.,  and  its  average  grade  is  2.00  per 
cent,  copper. 

AT  MORENCI,  ARIZONA,  the  upper  limit  o^B^clialcocite  zone  in 
the  disseminated  deposits  under  Copper  Mountain  is  represented 
by  a  curve  somewhat  less  convex  than  the  contour  of  the  surface. 
The  thickness  of  the  chalcocite  zone  is  somewhat  over  200  ft., 
although  directly  below  the  summit  it  reaches  300  ft.  in  thick- 
ness.2 The  depth  of  leached  material  in  this  district  varies  from 
a  few  feet  only  to  over  200  ft.  The  average  grade  of  the  ore  now 
being  mined  is  probably  between  2.00  and  2.5  per  cent,  copper. 

Chalcopyrite  Enrichments. — Chalcopyritic  enrichments  are 
not  of  so  common  occurrence  as  enrichments  of  chalcocite,  but 
form  important  deposits  in  certain  districts.  Secondary  chal- 
copyrite  is  commonly  associated  with  bornite,  and  also  with 
limonite. 

AT  BINGHAM,  UTAH,S  the  enrichment  of  the  Highland  Boy  ore- 
body  is  chiefly  chalcopyritic.  Here  the  carbonate  and  oxide 
ore  passes  into  an  enriched  zone  characterized  by  chalcopyrite, 
tarnished  and  coated  with  bornite  and  seamed  with  limonite: 
covellite  is  occasionally  an  associated  mineral.  Below  this  zone 
of  sulphide  enrichment  the  primary  ore  is  found,  consisting  of 
lean  cupriferous  pyrite.  The  transition  between  the  enriched 
ore  and  the  overlying  oxidized  ore  is  gradual,  as  is  also  the 
change  to  the  underlying  primary  ore. 

AT  SAN  ANTONIO  DE  LA  HUERTA,  SONORA,  the  primary  solu- 
tion-breccia ore-bodies  appear  to  have  received  a  chalcopyritic 

1  Henry  Krumb,  "First  Annual  Report  of  the   Inspiration   Copper  Co," 

2  Waldemar  Lindgren,  P.  P.  43,  U.  S.  G.  S.,  p.  204. 

3  J.  M.  Boutwell,  P.  P.  38,  U.  S.  G.  S.,  p.  223. 


SECONDARY  ORES  AND  ORE-SHOOTS  185 

enrichment.  The  chalcopyrite  is  here  associated  with  limonite 
and  small  quantities  of  copper  carbonates  and  silicates;  the  en- 
richments occur  at  slight  depth  below  the  surface. 

The  Enrichment  of  Gold  and  Silver  with  Copper. — Under  certain 
conditions,  depending  upon  the  presence  of  proper  solvents  and 
the  absence  of  precipitants,  gold  and  silver  migrate  and  form 
enrichments  in  company  with  copper.  Surface  waters  contain- 
ing chlorides  precipitate  silver  in  the  oxidized  zone  as  chloride 
in  many  instances,  and  the  enrichment  of  silver  with  the  second- 
ary copper  sulphides  is  thus  in  part  prevented.  Not  infre- 
quently, however,  only  a  part  of  the  silver  is  precipitated  as 
residual  ore,  while  the  remainder  descends  with  the  copper  to  the 
water  level.  The  enrichment  of  gold  in  copper  deposits  is  like- 
wise variable,  and,  perhaps,  takes  place  less  frequently  even  than 
the  enrichment  of  silver,  being  left  in  most  cases  as  residual  ore 
in  the  oxidized  zone.  Theoretically,  silver  should  be  precipitated 
higher  up  in  the  zone  of  secondary  enrichment  than  copper,  as 
its  affinity  for  sulphur  is  greater.  The  copper  in  enriched  copper- 
silver-gold  ores  is  commonly  in  the  form  of  chalcocite,  although  it 
also  occurs  in  such  association  as  chalcopyrite  and  enargite. 

AT  Rio  TINTO,  SPAIN/  the  oxidized  zone,  from  30  to  150  ft. 
deep,  carries  from  35  to  50  per  cent,  iron  with  traces  of  copper, 
arsenic  and  sulphur.  Directly  beneath  the  gossan  and  resting 
on  the  pyritic  ore  in  certain  deposits  there  occurs  a  bed  from 
4  to  8  in.  thick  of  earthy,  porous  material  that  carries  from 
$10  to  $20  gold  and  about  40  oz.  of  silver.  The  upper  part  of  the 
pyritic  zone  is  enriched  by  a  net-wrork  of  seamlets  of  chalcocite, 
bornite  and  chalcopyrite,  which  minerals  occur  in  progressively 
less  quantity  with  increasing  depth.  The  pyritic  ore  directly 
below  the  gossan  carries  from  4  to  5  per  cent,  copper;  at  a  depth 
of  from  200  to  230  ft.  the  copper  averages  about  2  per  cent.;  at 
330  ft.  about  1.5  per  cent. ;  at  from  425  to  460  ft.  about  1  per  cent. 
The  primary  ore  consists  of  pyrite  with  a  little  quartz  and  chal- 
copyrite. 

AT  SILVERTON,  COLORADO,  the  enrichment  of  silver  with  cop- 

1  Beck- Weed,  "Nature  of  Ore  Deposits,"  p.  485. 


186  .  EXAMINATION  OF  PRO&PECTS 

per  is  well  illustrated.  According  to  F.  L.  Ransome:1  "The  ore 
first  struck,  in  some  cases  at  the  surface,  consisted  chiefly  of 
argentiferous  galena.  At  a  depth  varying  somewhat  in  differ- 
ent mines,  but  which  appears  commonly  to  have  been  less  than 
200  ft.,  the  galena  ore  changed  to  an  ore  consisting  chiefly  of 
highly  argentiferous  stromeyerite,  silver  and  copper  glance. 
At  a  still  greater  depth,  commonly  at  about  500  ft.,  the  stro- 
meyerite changed  to  argentiferous  bornite,  still  deeper  to  chal- 
copyrite  and  pyrite,  and  finally  to  low-grade  auriferous  and 
argentiferous  pyrite.  These  changes  were  more  or  less  irregular 
and  overlapping,  pyrite,  for  example,  was  found  at  nearly  all 
levels,  and  bunches  of  galena  were  met  with  far  below  the  levels 
at  which  this  mineral  ceased  to  be  the  dominant  ore.  Small, 
rich  streaks  of  bornite  were  also  found,  with  chalcopyrite  and 
pyrite,  below  the  levels  at  which  it  occurred  in  large  masses.'7 
According  to  Schwartz2  the  rich  parts  of  the  ore-bodies  were  in 
every  case  associated  with  open,  water-bearing  fissures. 

The  Enrichment  of  Silver. — The  primary  ores  of  silver  are 
readily  attacked  by  oxidation,  and  in  the  presence  of  acid  sul- 
phate solutions,  yield  their  silver  as  soluble  sulphate;  in  this 
state  silver  may  descend  to  form  secondary  enrichments  at  the 
water  level,  where  it  is  readily  precipitated  by  other  sulphides. 
The  leaching  of  the  upper  parts  of  silver  deposits  is  similar  to  the 
corresponding  alteration  of  copper  deposits,  with  the  added 
factor  that  a  certain  proportion  of  the  silver  is  likely  to  be  pre- 
cipitated in  the  oxidized  zone  as  chloride,  or  as  native  silver. 

The  complex  sulphur,  arsenic  and  antimony  compounds  of 
silver  occur  as  both  primary  and  secondary  minerals,  and  it  is 
often  difficult  to  determine  to  which  process  they  should  be  as- 
signed. Where  copper  occurs  with  silver  in  deposits  worked  over 
by  surface  agencies  the  two  metals  frequently  migrate  and  pre- 
cipitate together,  although  the  tendency  is  for  a  certain  propor- 
tion of  the  silver  to  be  left  behind  as  residual  minerals  in  the 
oxidized  zone  and  for  the  secondary  sulphide  of  silver  to  occur 

1  F.  L.  Ransome,  Bull.  182,  U.  S.  G.  S.,  p.  137. 
2 Trans.,  A.  I.  M.  E.,  XVIII,  p.  144. 


SECONDARY  ORES  AND  ORE-SHOOTS  187 

near  the  top  of  the  zone  of  enrichment.  Many  of  the  great 
bonanzas  of  silver  ores  in  arid  climates  are  probably  secondary 
enrichments. 

Stibnite  is  of  frequent  occurrence  in  silver  ores  in  many  dis- 
tricts in  Mexico;  it  is  typically  a  primary  mineral,  and  the  man- 
ner of  intergrowth  of  the  silver  minerals  with  it  may  indicate 
their  primary  or  secondary  origin. 

In  the  investigation  of  deposits  of  silver  ores  where  secondary 
enrichment  is  suspected,  it  should  be  borne  in  mind  that  a 
relatively  small  body  of  rich  silver  ore  may  yield  important 
economic  return;  no  such  general  leaching  of  rocks  or  complete 
alteration  of  primary  sulphides,  therefore,  should  be  required 
upon  which  to  base  an  expectation  of  important  deposits  as  is 
the  case  with  secondary  copper  ores. 

AT  THE  PROMONTORIO  MINE,  DURANGO,  MEXICO,*  the  primary 
ore  consists  chiefly  of  quartz,  galena,  and  zincblende,  a  little  pyrite, 
and  subordinate  chalcopyrite.  The  oxidized  vein-filling  con- 
sists of  quartz,  kaolin,  hematite,  wad,  and  limonite,  with  occa- 
sional films  of  malachite,  linarite  and  the  remains  of  sulphides. 
The  minerals  that  have  contributed  to  secondary  enrichment  are 
native  silver,  chalcocite,  and  a  little  chalcopyrite.  The  second- 
ary enrichments  are  contained  in  the  oxidized  parts  of  the  vein, 
and  in  the  country  rock  of  the  walls  and  horses.  The  primary 
ore-shoots  are  distinguishable  by  their  comparatively  high  con- 
tent of  sulphides,  by  their  lack  of  secondary  minerals,  and  by 
their  habit  of  being  cut  off  by  faults  unless  occurring  in  unfaulted 
parts  of  the  vein.  The  secondary  ore-shoots  are  recognized  by 
their  low  content  of  sulphides,  by  the  presence  in  their  richer 
parts  of  the  secondary  minerals,  native  silver,  chalcocite  and 
chalcopyrite,  and  by  their  tendency  to  follow  closely  well-de- 
fined faults.  Primary  ore-shoots  are  dominant  in  the  lower 
levels  and  the  secondary  ore-shoots  in  the  upper  levels. 

AT  GEORGETOWN,  COLORADO,2  the  silver  lead  deposits  show 
evidence  of  rearrangement  by  surface  agencies.  The  zone  of 

1  F.  C.  Lincoln,  Trans.,  A.  I.  M.  E.,  XXXVIII,  p.  740. 

2  Spurr  and  Garrey,  P.  P.  63,  U.  S.  G.  S.,  p.  143. 


188  EXAMINATION  OF  PROSPECTS 

complete  oxidation  in  these  veins  usually  extends  from  the  surface 
to  a  depth  of  from  5  to  40  ft.  only.  The  oxidized  material  is  a 
brown  clay;  it  is,  in  general,  rich  ore,  containing  several  hundred 
ounces  of  silver.  Below  this  oxidized  material,  and  mixed 
with  the  lower  part  of  it,  occur  friable,  locally  powdery,  black 
sulphides  and  bunches  of  secondary  galena.  These  pulverulent 
sulphides  are  rich  ores  also,  containing  relatively  large  quan- 
tities of  silver  and  of  lead,  and  more  gold  than  occurs  at  greater 
depths.  The  soft  sulphides  are  found  chiefly  in  cracks  and 
along  water  courses,  and  are  of  secondary  origin:  they  persist  to 
considerable  depths  below  the  surface,  but  in  decreasing  quan- 
tities. Below  the  zone  of  soft  secondary  sulphides,  and  irregu- 
larly within  its  lower  part,  occur  secondary  polybasite,  argen- 
tiferous tetrahedrite,  and  ruby  silver,  which  in  quantity  diminish 
steadily  but  irregularly  with  increasing  depth.  The  best  ore 
in  most  of  these  veins  has  been  found  within  500  ft.  of  the 
surface,  but  locally  it  extends  down  to  700  or  800  ft.,  and  in 
one  case  tft  1000  ft.  below  the  surface.  Much  secondary  pyrite 
and  siderite  occur  in  the  upper  parts  of  these  veins.  The  pri- 
mary ore  carries  from  20  to  30  oz.  of  silver  and  the  enriched 
ore  from  200  to  300  oz.  per  ton.  The  primary  ore  consists  of 
galena,  zincblende  and  cupriferous  pyrite  in  a  gangue  of  quartz, 
and  the  carbonates  of  iron,  manganese,  magnesia  and  lime. 

AT  GEORGETOWN,  COLORADO/  in  the  Bismark  ore-shoot,  the 
oxidized  ores  of  brown  clayey  material  carry  several  hundred 
ounces  of  silver  and  extend  to  a  depth  of  about  40  ft.  Mixed 
with  these  ores  and  extending  from  200  to  300  ft.  deeper,  most 
prominently  along  water  courses,  soft  black  secondary  sulphides 
are  found.  These  ores  consist  chiefly  of  galena  and  zincblende, 
and  carry  several  hundred  ounces  of  silver  per  ton.  The  primary 
ore  is  composed  of  zincblende  and  galena  with  some  pyrite,  in  a 
gangue  of  quartz,  siderite,  subordinate  barite,  and  calcite. 

AT  LAKE  CITY,  COLORADO, 2  the  primary  ore  of  the  silver- 
bearing  fissure  veins  consists  of  galena,  tetrahedrite,  chalcopy- 

1  Spurr  and  Garrey,  citing  B.  B.  Lawrence,  P.  P.  63,  U.  S.  G.  S.,  p.  190. 

2  J.  D.  Irving,  Bull.  260,  U.  S.  G.  S.,  p.  81. 


SECONDARY  ORES  AND  ORE-SHOOTS  189 

rite,  zincblende  and  pyrite  in  a  gangue  of  quartz,  rhodonite, 
rhodochrosite  and  barite.  In  the  upper  parts  of  the  veins 
secondary  ruby  silver  and  argentite  are  found  associated  with 
anglesite,  cerussite,  limonite  and  pyrolusite.  The  values  in  the 
upper  parts  of  the  veins  are  commonly  high,  and  bonanzas  of 
ruby  silver  are  found. 

AT  BROKEN  HILL,  N.  S.  W.,1  the  oxidized  zone  is  largely  made 
up  of  "  kaolin  ore,"  which  carries  oxidized  minerals  of  iron, 
manganese,  lead  and  copper,  and  relatively  low  silver  values. 
In  the  lower  part  of  the  oxidized  zone  occur  the  "  dry  ores/'  which 
carry  the  antimonial  and  arsenical  sulphides  of  silver,  polybasite, 
stromeyerite,  dycrasite,  proustite,  pyrargyrite,  and  stephanite. 
Beneath  the  dry  ores  occurs  a  thin  layer,  from  3  in.  to  6  ft.  in 
thickness,  of  sooty  black  sulphides,  which  carry  as  much  as  250 
oz.  of  silver  and  12  per  cent,  copper.  This  layer  rests  upon  the 
primary  ores,  which  consist  of  an  intimate  mixture  of  argentif- 
erous galena  and  blende,  in  a  gangue  of  quartz,  garnet,  rhodo- 
nite, and  feldspar,  with  chalcopyrite,  arsenopyrite,  wulfenite 
and  fluorite  as  accessory  minerals.  The  primary  ores  carry 
from  5  to  36  oz.  of  silver,  7  to  50  per  cent,  lead,  and  14  to  30  per 
cent.  zinc. 

AT  NEIHART,  MONTANA,*  superficial  alteration  is  not  marked, 
and  no  great  zones  of  carbonate  or  oxidized  ores  ore  found:  the 
deepest  general  oxidation  in  the  district  reaches  170  ft.  below 
the  outcrop,  but  locally  oxidation  extends  as  pipes  and  along 
drainage  fissures  to  greater  depths.  The  secondarily  enriched 
ores  carry  polybasite,  pyrargyrite  and  argentite  as  crusts  lining 
cavities,  as  thin  seamlets  through  the  primary  ores,  and  as 
sooty  sulphides  associated  with  maganese  oxides  in  the  oxidized 
zone.  The  secondary  minerals  also  occur  in  the  fractures  in  the 
shattered  country  rock.  The  primary  ore  contains  galena, 
zincblende,  and  pyrite  in  a  gangue  of  quartz  and  barite. 

The  Enrichment  of  Gold. — Gold  is  readily  soluble  in  the  pres- 
ence of  nascent  chlorine,  which  is  produced  by  the  action  of 

1  S.  F.  Emmons,  Trans.,  A.  I.  M.  E.,  XXX,  p.  204,  quoting  Jaquet. 

2  W.  H.  Weed,  Trans.,  A.  I.  M.  E.,  XXX,  p.  435. 


190  EXAMINATION  OF  PROSPECTS 

sulphuric  acid  upon  chlorides  in  the  presence  of  the  oxide  of 
manganese,  all  of  which  compounds  are  frequently  present  in  the 
zone  of  oxidiation.  Gold  is  also  soluble  in  solutions  of  ferric 
hydrate,  sodium  sulphide,  sodium  carbonates  and  other  reagents 
formed  in  nature.  Gold  is  precipitated  readily  by  pyritic  sul- 
phides and  by  organic  matter,  and  when  in  solution  with  ferric 
hydrate  is  precipitated  upon  the  reduction  of  this  compound  to 
the  ferrous  salt,  a  change  likely  to  take  place  at  water  level. 
Gold  also  migrates  and  precipitates  with  the  antimonial  sul- 
phides of  silver.1 

The  relatively  small  bulk  of  gold  as  compared  with  its  asso- 
ciated gangue,  commonly  resistant  quartz,  tends  to  protect  the 
gold  from  solution  and  also  renders  it  difficult  to  distinguish  a 
primary  gold  ore  from  one  of  secondary  origin.  The  association 
of  rich  ores  with  post-mineral  fissures,  the  presence  of  films  of 
oxidized  minerals  through  the  ore,  and  the  shape  of  the  ore 
shoots  are  the  most  reliable  criteria  of  secondary  origin.  Second- 
ary ore-shoots  are  likely  to  be  longer  horizontally  than  they  are 
deep  in  deposits  of  uniform  permeability,  while  primary  ore 
shoots  are  commonly  deeper  than  they  are  long. 

A  majority  of  gold  deposits  are  larger  and  richer  near  the 
surface  than  in  depth,  and  their  ore-shoots  often  carry  films  of 
oxidized  minerals,  such  as  dendritic  oxide  of  manganese,  or  an 
incipient  oxidation  of  the  associated  sulphides,  far  below  the 
zone  of  weathering.  The  high-grade  ores  found  under  these 
conditions  are  probably  the  result,  in  part,  of  secondary  enrich- 
ment. Gold  is  less  easily  dissolved  under  average  conditions 
than  are  most  other  metals,  and  is  frequently  left  behind  in  the 
upper  parts  of  ore-deposits  in  which  secondary  enrichments  of 
other  metals  have  formed  at  the  water  level. 

It  is  not  uncommon  to  find  rich  bodies  of  gold  ore  at  and  just 
beneath  the  outcrops  of  gold  veins,  due  probably  to  the  several 
processes  of  mechanical  concentration  during  weathering,  resid- 
ual concentration  through  the  removal  of  soluble  constituents, 
and  also  to  some  extent  to  solution,  migration  and  enrichment. 

1  W.  H.  Weed,  Trans.,  A.  I.  M.  E.,  XXX,  p.  432. 


SECONDARY  ORES  AND  ORE-SHOOTS  191 

AT  THE  RUBY  MINE,  MONTANA/  an  important  secondary  ore- 
body,  in  which  the  values  were  about  equal  in  gold  and  silver, 
occurred  along  a  post-mineral  fault.  The  gold  and  the  silver 
appear  to  have  migrated  together  in  antimonial  sulphides, 
notably  pyrargyrite,  which  occurred  as  coatings  on  boulders  of 
decomposed  rhyolite,  and  lining  cavities  in  the  zone  of  enrich- 
ment. 

IN  THE  GRANITE  VEIN,  MONTANA/  which  is  described  in  a  pre- 
ceding paragraph,  the  low-grade  ore  of  the  leached  zone  carries 
but  little  gold;  the  enriched  oxide  ore  which  occurs  at  the  base  of 
the  oxidized  zone  carries  from  $5  to  $16,  the  underlying  enriched 
sulphide  ore  from  $4  to  $8,  and  the  primary  ore  from  $1.50  to  $3. 
The  accompanying  silver  values  are  concentrated  in  greater 
degree  than  the  gold. 

AT  MONTE  CRISTO,  WASHINGTON/  there  is  no  zone  of  complete 
oxidation  and  the  sulphides  frequently  outcrop.  Partial  oxida- 
tion extends  to  a  depth  of  perhaps  10  ft.  The  distribution 
of  the  sulphide  minerals  indicates  rearrangement  by  surface 
agencies.  The  upper  zone  is  characterized  by  galena,  gold, 
and  silver,  and  its  ores  average  $19.00  in  gold  and  12  oz.  of 
silver.  The  lower  limit  of  the  galena  zone  follows  the  contour  of 
the  surface  at  depths  of  from  100  to  150  ft.  Below  this  occur 
less  regular,  but  still  definite,  zones  that  are  characterized 
respectively  by  zincblende,  chalcopyrite,  and  arsenopyrite  with 
pyrite.  The  ores  of  the  last-named  zone  average  $12  gold  and 
7  oz.  of  silver. 

The  Enrichment  of  Lead. — Lead  is  less  easily  soluble  than 
most  other  metals:  upon  oxidation  it  alters  first  to  the  sulphate, 
anglesite,  and  then  to  the  carbonate,  cerussite.  The  carbonate, 
apparently,  may  also  form  directly  from  the  sulphide.  Owing 
to  the  relative  insolubility  of  these  minerals  lead  is  dissolved 
slowly  and  is  readily  precipitated  and  the  process  of  enrichment 

1  W.  H.  Weed,  Trans.,  A.  I.  M.  E.,  XXX,  p.  433. 

2  W.  H.  Emmons,  Bull.  315,  U.  S.  G.  S.,  p.  39. 

3  J.  E.  Spurr,  Twenty-second  Annual  Report,  U.  S.  G.  S.,  and  "Geology 
Applied  to  Mining,"  p.  280. 


192  EXAMINATION  OF  PROSPECTS 

is  a  slow  one.  Long  continued  action  of  surface  waters,  how- 
ever, with  repeated  solution  and  precipitation  may  produce  a 
zone  of  lead  enrichment  higher  up  in  a  deposit  than  the  enrich- 
ments of  associated  metals  such  as  copper  and  zinc,  whose  second- 
ary sulphides  are  commonly  found  at  the  water  level.  Galena 
is  slightly  soluble  in  water,  and  in  solutions  containing  sodium 
sulphide.  The  sulphate  of  lead,  anglesite,  is  slightly  soluble 
in  water,  and  the  carbonate,  cerussite,  is  soluble  in  carbonated 
waters.  Galena  is  precipitated  by  hydrogen  sulphide  produced 
by  the  reactions  of  solutions  upon  sulphides,  by  organic 
matter,  and  probably  when  migrating  as  the  sulphide,  by  direct 
replacement  of  calcite. 

While  in  most  deposits  the  upper  lead  bearing  zone  is  in  large 
part  made  up  of  residual  galena,  anglesite  and  cerussite,  migra- 
tion and  enrichment  have  probably  assisted  in  concentrating 
these  minerals.  The  migration  of  lead  in  the  presence  of  acid 
sulphate  .solutions  is  apparently  more  difficult  and  less  com- 
plete than  in  certain  limestone  deposits  where  carbonated  waters 
have  probably  acted  as  the  solvents. 

The  examples  of  secondary  enrichments  of  galena  are  neither 
numerous  nor  clear,  the  usual  occurrence  of  lead  minerals  in 
deposits  worked  over  by  surface  agencies  being  as  residual 
minerals. 

AT  MONTE  CRISTO,  WASHINGTON,  as  described  in  a  preceding- 
paragraph,  the  upper  part  of  the  sulphide  zone  is  characterized 
by  galena,  the  lower  limit  of  which  follows  roughly  the  contour 
of  the  surface,  indicating  a  concentration  by  surface  agencies. 

AT  GEORGETOWN,  COLORADO/  in  the  veins  of  the  Freeland 
group  galena  was  more  abundant  in  the  upper  few  hundred  feet, 
while  but  small  quantities  of  this  mineral  is  contained  by  the 
characteristic  pyritic  ore.  In  the  zone  of  relatively  abundant 
galena,  brown  carbonates  (siderite,  rhodochrosite,  and  some 
barite),  were  abundant,  as  was  also  tetrahedrite,  which  occurred 
in  cracks  through  the  older  sulphides.  Both  the  carbonates  and 
tetrahedrite  were  in  places  clearly  of  secondary  origin. 

1  Spurr  and  Carrey,  P.  P.  63,  U.  S.  G.  S.,  p.  149. 


SECONDARY  ORES  AND  ORE-SHOOTS  193 

IN  THE  UPPER  MISSISSIPPI  VALLEY  1  deposits  of  zincblende  and 
galena  associated  with  marcasite,  pyrite  and  subordinate  chalco- 
pyrite  occur  in  unaltered,  flat  lying  dolomites  and  limestones 
far  from  any  known  igneous  rocks,  and  apparently,  in  most 
cases,  unconnected  with  any  important  fissure.  In  form  these 
ore-bodies  occur  in  vertical  crevices,  in  irregular  ore-bodies  at 
the  juncture  of  an  ore  bearing  horizon  with  a  vertical  fissure, 
in  pitches  and  flats,  and  in  thin,  flat  lying  disseminations.  The 
ore-bodies  are  closely  related  to  certain  very  shallow  structural 
basins,  and  to  certain  shale  beds  known  locally  as  oil  rock, 


FIG.  75. — Diagram  illustrating  type  of  "pitch  and  flat"  deposit  of  the 
Upper  Mississippi  Valley,  a,  Flats;  b,  pitches;  c,  vertical  crevice.  After 
Grant. 

which  contain  a  large  percentage  of  fossil  gum.  These  ore- 
bodies  are  believed  by  many  to  represent  concentrations  by 
descending  waters  of  sparsely  disseminated  mineralizations; 
their  genesis  must  be  considered  doubtful.  The  common  verti- 
cal distribution  of  the  several  ore  minerals  is:  galena  in  the 
uppermost  zone,  below  which  is  found  zinc  carbonate  ore,  under- 
lain in  turn  by  zincblende  ores  below  the  water  level,  while  the 
lowest  zone  is  pyritic;  the  several  zones  commonly  merge  into 
each  other.  The  localization  of  the  ores  appears  to  be  con- 
nected with  the  water  level;  lead  ores  occur  above  the  water 
level,  zinc  carbonate  from  slightly  above  to  slightly  below, 
and  the  zincblende  below  but  near  to  the  water  level.  A  second- 
ary distribution  is  thus  indicated. 

1  H.  F.  Bain,  Bull.  294,  U.  S.  G.  S.,  p.  46. 
13 


194  EXAMINATION  OF  PROSPECTS 

While  scanty  occurrences  of  zincblende  and  galena  are  known 
in  both  the  overlying  and  underlying  rocks,  the  important 
mineralizations  are  confined  to  the  Galena  dolomite,  and  in 
those  districts  where  this  rock  has  been  largely  eroded  they  also 
occur  in  the  underlying  Platteville  formation:  while  ore-deposits 
are  found  in  all  horizons  of  these  beds  certain  strata  are  locally 
more  likely  to  contain  ore  than  others.  The  ore-deposits  are  in 
the  main  confined  to  very  shallow  synclines.  The  aforemen- 
tioned "oil  rock"  is  a  brown  or  black  shale  varying  in  thickness 
from  6  to  8  ft.  down  to  a  few  inches;  it  contains  thin  lenses  of 
dolomite  and  the  shaly  material  commonly  does  not  form  single 
bands  more  than  one  foot  in  thickness.  This  bed  contains  fossil 
gum  that  appears  to  have  been  active  in  precipitating  the  ore 
minerals.  Pitches  and  flats  are  commonly  developed  above  the 
oil  rock,  which  itself  frequently  contains  disseminated  ores.  The 
oil  rock  is  seldom  seen  in  any  considerable  thickness  outside  of 
the  mines,  though  it  is  not  of  such  a  nature  as  to  be  especially 
subject  to  weathering. 

The  Enrichment  of  Zinc. — Zincblende  is  readily  attacked  by 
oxidizing  solutions  and  the  resulting  sulphate  is  very  soluble. 
Zinc  is  usually  completely  leached  from  the  oxidized  zones  of 
most  deposits,  but  sulphide  enrichments  in  igneous  rocks  are 
comparatively  rare;  in  sedimentary  rocks,  however,  migrating 
solutions  carrying  zinc  frequently  replace  certain  beds  of  the 
country  rock  with  oxidized  zinc  minerals.  Such  deposits, 
even  when  of  important  extent,  are  difficult  to  detect,  as  the 
replacement  commonly  retains  the  structure  and  to  some  ex- 
tent the  color  of  the  replaced  rock,  while  the  low  specific  gravity 
of  these  minerals  does  not  call  attention  to  their  presence. 
Zincblende,  under  certain  conditions,  apparently  migrates  with 
ease  in  limestone. 

IN  THE  UPPER  MISSISSIPPI  VALLEY  secondary  enrichments  of 
zincblende  are  common  in  the  deposits  in  limestone  and  dolomite. 
The  zincblende  occurs  just  below  the  water  level  and  is  usually 
associated  with  marcasite  and  some  galena.  This  zone  passes  in 
depth  into  ores  that  carry  abundant  marcasite  with  but  little  zinc. 


SECONDARY  ORES  AND  ORE-SHOOTS  195 

AT  LEADVILLE,  COLORADO,  the  zinc  is  commonly  leached  from 
the  oxidized  ores  near  the  surface  and  forms  large  replacement  de- 
posits of  oxidized  minerals  in  depth,  the  existence  of  which  was  not 
suspected  for  many  years. 

AT  PACHUCA,  MEXICO/  the  silver  veins  are  impoverished  in 
depth  by  large  quantities  of  barren  zincblende. 

The  Enrichment  of  the  Lesser  Metals. — Under  certain  con- 
ditions, nickel,  cobalt,  arsenic,  antimony,  tin  and  cadmium  appear 
to  be  capable  of  solution,  migration  and  enrichment. 

The  Migration  of  Gangue  Minerals. — The  solution,  migration, 
and  occasionally  the  precipitation,  of  gangue  minerals  are  im- 
portant processes  in  the  rearrangement  of  ore-deposits  by  sur- 
face agencies.  The  residual  concentration  of  gold  in  the  oxidized 
zone  is  the  result  of  the  solution  and  removal  of  associated  gangue 
minerals  and  sulphides. 

The  carbonates  and  alumina  appear  to  be  especially  susceptible 
to  solution  and  removal,  while  under  certain  conditions  quartz 
is  a  mobile  mineral. 

The  oxides  of  manganese  are  apparently  soluble  in  solutions 
of  ferrous  sulphate;2  the  presence  of  dendritic  oxide  of  man- 
ganese in  many  deep  deposits  is  probably  the  result  of  the 
migration  of  this  compound  in  solution. 

1  Waldemar  Lindgren,  Trans.,  A.  I.  M.  E.,  XXX,  p.  650. 

2  F.  W.  Clarke,  quoting  F.  P.  Dunnington,  Bull.  330,  U.  S.  G.  S.,  p.  458. 


CHAPTER  XI 
OUTCROPS 

In  the  examination  of  an  undeveloped  prospect  a  decision 
must  be  arrived  at  from  an  inspection  of  the  outcrops  and  the 
exposures  in  a  few  shallow  pits.  Prospects  that  are  offered  for 
sale  rarely  expose  any  important  quantity  of  payable  ore,  work 
having  usually  been  stopped  when  the  immediate  exploration  no 
longer  yielded  favorable  results.  It  should  be  borne  in  mind, 
furthermore,  that  a  great  majority  of  prospects  have  been  ex- 
amined many  times,  and  if  of  conspicuous  promise,  would  have 
been  acquired  for  development.  In  most  cases  the  problem 
for  the  engineer,  therefore,  is  to  discover  the  traces  of  a  valuable 
mineralization  that  has  disappeared  through  solution,  or  the 
conditions  that  indicate  the  possibility,  or  probability,  of  under- 
lying secondary  enrichments. 

The  available  data  being  meager,  every  feature  should  be  the 
subject  of  careful  study — the  condition  of  the  outcrop,  the 
structural  relationships,  the  associations  of  minerals,  the  altera- 
tions of  wall  rocks,  the  chances  for  underlying  enrichments,  and 
so  forth,  as  well  as  the  actual  assay  value  of  the  material  exposed. 

It  is  frequently  advisable  to  spend  the  time  and  money  neces- 
sary to  have  trenches  dug  at  various  significant  points  along  a 
promising  outcrop;  such  exposures,  even  if  the  depth  attained  is 
slight,  often  disclose  conditions  that  are  not  apparent  at  the 
actual  surface,  as  for  example,  the  existence  of  soft  minerals 
and  the  distribution  of  the  various  minerals  through  the  mass  of 
the  deposit.  Trenches  also  permit  samples  to  be  taken  from 
points  that  were  not  accessible  during  former  examinations. 

The  Relation  between  Length  of  Outcrop  and  Persistency  of 
Vein  in  Depth. — Strong,  persistent  outcrops  of  uniform  width 
may  be  taken  to  indicate  the  probable  size  and  character  of  the 
underlying  vein,  whose  persistency  in  depth  is  likely  to  be 

196 


OUTCROPS        ^  197 

proportional  to  the  length  of  its  outcrop.  Fissures  that  may  be 
traced  for  long  distances  on  the  surface  are  commonly  found  to 
be  equally  persistent  in  depth,  while  short,  branching  and  irreg- 
ular outcrops  are  usually  indicative  of  similar  irregularities  in 
the  underlying  deposits.  Outcrops  that  comprise  a  series  of  large 
irregular  masses,  perhaps  connected  by  narrower  veins,  are  in 
general  likely  to  become  smaller  in  depth. 

The  Relation  between  Size  of  Outcrop  and  Width  of  Vein  in 
Depth. — In  a  vein  or  deposit  that  is  harder  and  more  resistant 
than  the  enclosing  rocks,  erosion  is  likely  to  be  halted  at  the 
widest  part,  that  part  offering  the  greatest  resistance  to  erosion 
and  the  greatest  protection  to  the  adjoining  rocks.  Such  out- 
crops, therefore,  are  likely  to  be  larger  than  the  average  of  the 
underlying  deposit.  The  converse  of  this  condition  is  also  true. 
A  vein  or  deposit  that  is  softer  and  less  resistant  to  erosion  than 
the  enclosing  rocks  is  likely  to  become  larger  in  depth.  Erosion 
tends  to  remove  the  surface  of  such  a  deposit  faster  than  the 
adjoining  wall  rocks,  forming  a  depression,  the  walls  of  which 
tend  to  close  together  upon  the  removal  of  the  intervening  soft 
material;  occasionally,  a  narrow  and  inconspicuous  gouge-filled 
fissure  is  the  only  surface  indication  of  an  important  vein  of  soft 
minerals. 

Brecciation  and  Post-mineral  Fracturing. — In  disseminated 
deposits  the  most  intense  primary  mineralization  is  likely  to  be 
connected  with  a  thorough  brecciation,  as  are  also  disseminated 
enrichments.  The  study  of  post-mineral  fracturing,  therefore, 
is  of  as  much  importance  in  the  investigation  of  these  deposits 
as  is  the  study  of  the  minerals  of  the  outcrops  themselves. 

Meandering  of  Outcrops  on  Hillsides. — The  outcrop  of  a  vertical 
vein  is  a  straight  line,  whatever  the  slope  of  the  surface,  and  the 
outcrop  of  a  horizontal  bed  follows  a  contour  around  all  hillsides. 
A  vein  of  intermediate  dip,  however,  outcrops  to  the  right  or 
to  the  left  of  its  strike  at  any  horizon  in  accordance  with  its 
dip  and  the  slopes  of  the  surface.  An  idea  may  be  obtained  in 
the  field  as  to  the  dip  of  a  vein  by  paralleling  a  book  or  other  plane 
surface  with  several  parts  of  the  outcrop  at  different  elevations, 


198 


EXAMINATION  OF  PROSPECTS 


while  in  mapping,  simple  trigonometric  calculations  or  graphic 
solutions  will  give  the  heights  or  horizontal  positions  of  the 
vein  at  any  desired  point:  contouring  is  expensive  and  is  usually 
unnecessary. 

-  Down  Hill  Creep. — An  outcrop  situated  on  a  steep  hillside  is 
likely  to  overturn  in  the  direction  of  the  slope  of  the  hill;  loose 
fragments  of  the  partially  disintegrated  outcrop  become 


FIG.  76. — Outcrop  of  a  vein  at  Bullfrog,  Nevada,  showing  surface  overturn- 
ing and   down-hill   creep.     After  Ransome. 

mingled  with  the  surface  material,  and  when  at  some  distance 
from  the  parent  mass  are  widely  separated  and  constitute  "float." 
The  apparent  dip  of  a  vein  that  outcrops  along  a  steep  hillside 
is,  therefore,  likely  to  be  flatter  than  its  true  dip  below  the  me- 
chanical influence  of  erosion. 

The  Topographic    Expression   of    Mineralization. — Mineraliza- 
tion   occasionally    finds    local    expression    in    the    topography. 


OUTCROPS 


199 


Silicified  areas  and  resistant  outcrops  may  form  prominent  ridges 
or  knobs,  and  soft  or  altered  rocks  may  result  in  depressions  or 
saddles  in  the  ridges.  Faults  are  occasionally  prominent 
topographic  features  where  one  wall  is  notably  more  resistant  to 


FIG.  77. — Fault  plane  developed  into  a  scarp  by  erosion,  Globe,  Arizona; 
the  rock  on  the  left  (hanging-wall)  is  quartzite,  that  on  the  right  (foot-wall) 
is  granite.  After  Ransome. 

erosion  than  the  other,  but,  in  general,  unless  of  great  displace- 
ment, faults  are  not  likely  to  be  represented  in  the  topography. 
Different  kinds  of  rocks  upon  weathering  produce  characteristic 
topographic  outlines,  as,  for  example,  the  plateau-like  hills  and 


200  EXAMINATION  OF  PROSPECTS 

steep  talus  slopes  of  horizontal  sedimentary  beds,  or  the  ragged 
outlines  common  in  areas  of  volcanic  rocks.  While  topographic 
relief  is  rarely  significant  in  the  examination  of  mining  properties, 
it  may  be  a  valuable  guide  to  the  prospector,  as  many  mineralized 
areas  are  connected  with  low,  rounded  foot  hills  of  notably 
different  outline  from  the  general  relief  of  the  region. 
/  Outcrops  of  Deposits  Formed  at  Slight  Depth. — Deposits  formed 
at  slight  depth  below  the  surface  are  characteristically  irregular 
in  their  upper  part,  and  tend  to  consolidate  and  to  become 
structurally  more  regular  with  greater  depth. 

Porosity  of  Outcrops. — The  outcrops  of  ore-deposits  that  have 
yielded  to  oxidation  are  commonly  porous  and  cellular,  owing 
to  the  removal  of  certain  of  the  original  constituents  in  solution. 
Such  outcrops  are  favorable  in  that  they  indicate  solution  and 
possible  secondary  enrichments  of  the  dissolved  substances,  or, 
at  least,  the  original  presence  of  easily  dissolved  minerals  which 
in  their  unaltered  state  may  have  been  valuable.  A  massive, 
tight  outcrop,  on  the  other  hand,  is  usually  indicative  at  slight 
depth  of  the  typical  value  of  the  deposit,  which  has  presumably 
been  but  little  affected  by  secondary  agencies. 

Casts  in  Resistant  Gangue  Minerals. — Upon  the  oxidation  and 
leaching  of  an  ore  composed  of  sulphide  minerals  in  a  resistant 
gangue,  the  outcrop  is  likely  to  retain  the  casts,  or  open  spaces, 
left  by  the  removal  of  the  sulphides.  In  many  instances  out- 
crops exhibit  abundant  casts  of  minerals  that  have  completely 
disappeared,  and  whose  presence  originally  would  not  be  sus- 
pected without  a  close  examination  for  such  evidence. 

The  crystallization  of  pyrite  appears  to  be  interrupted  when 
it  contains  a  notable  proportion  of  copper.  Pure,  barren,  pyrite 
commonly  crystallizes  as  cubes  or  other  isometric  forms,  while 
chalcopyrite  is  commonly  distributed  in  an  irregular  manner; 
the  crystallization  of  cupriferous  pyrite,  in  general,  is  wavy  and 
irregular,  although  occasional  cubes  may  be  noted.  A  less 
strongly  emphasized  but  still  marked  relation  exists  between 
pure,  barren  galena  and  highly  argentiferous  galena.  This 
mineral  is  less  likely  to  be  well  crystallized  when  it  carries  an 


OUTCROPS  201 

important  proportion  of  silver.  An  examination  of  the  casts 
of  these  sulphides  in  a  leached  outcrop,  therefore,  is  likely  to 
yield  evidence  to  some  extent  in  regard  to  the  value  as  well  as  to 
the  kinds  of  the  sulphides  originally  present. 

The  Composition  of  Outcrops. — The  minerals  present  in  out- 
crops are  those  primary  constituents  of  the  ore  most  resistant  to 
solution,  and  the  most  resistant  of  the  products  of  alteration. 
The  most  resistant  of  the  primary  minerals  is  usually  quartz, 
which  commonly  forms  a  large  proportion  of  the  outcrops  of  the 
deposits  in  which  it  is  an  important  constituent.  Magnetite  and 
specularite  are  also  relatively  resistant  minerals.  Of  the  products 
of  oxidation  and  hydration  kaolin  and  limonite  are  the  most 
resistant,  and  are  commonly  found  in  or  close  beneath  the 
outcrop.  Sericite  likewise  is  resistant,  but  is  likely  to  be  changed 
to  kaolin  where  sulphuric  acid  solutions  are  prominent  in  accom- 
plishing the  surface  alteration. 

The  oxides  of  manganese  are  resistant,  and  are  frequently 
found  in  large  quantities  in  the  oxidized  parts  of  ore-deposits, 
even  those  in  which  manganese  forms  a  quite  subordinate  pro- 
portion of  the  primary  ore. 

Gold  is  a  resistant  mineral,  and  is  frequently  concentrated  at 
the  surface  and  in  the  oxidized  zone,  as  is  discussed  in  a  pre- 
ceding paragraph. 

The  more  resistant  of  the  ore  minerals  are  galena  and  its 
oxidation  products,  anglesite  and  cerussite,  chloride  of  silver, 
native  silver,  cuprite,  native  copper,  chrysocolla  and  to  a  lesser 
degree  the  carbonates  of  copper,  all  of  which  minerals  are  likely 
to  be  left  behind  as  residual  ores  during  solution  and  migration. 

In  copper  deposits  that  have  been  subjected  to  thorough  leach- 
ing and  alteration,  the  outcrops  are  likely  to  consist  of  soft  white 
kaolin  and  quartz,  carrying  at  the  surface,  perhaps,  trifling 
quantities  of  limonite,  manganese  oxide,  and  chrysocolla  as 
stains.  Such  outcrops  are  most  favorable  for  the  existence  of 
chalcocite  enrichments  in  depth. 

The  thorough  alteration  of  an  intensely  mineralized  mass  may 
remove  all  traces  of  the  primary  ore  minerals,  while  adjacent 


202  EXAMINATION  OF  PROSPECTS 

rocks  that  received  a  scanty  mineralization  only  and  so  remain 
relatively  unaltered,  often  retain  traces  of  the  primary  minerals 
and  so  furnish  a  clue  to  the  original  nature  of  the  principal 
deposit. 

The  Oxides  of  Iron  in  Gossan. — The  oxides  of  iron,  either 
massive  or  as  stain,  are  frequently  the  most  prominent  con- 
stituents of  outcrops  and  of  the  superficial  parts  of  ore-deposits 
that  have  suffered  oxidation.  The  condition  and  occurrence  of 
these  minerals  are  often  indicative  of  the  character  of  the 
mineralization  in  depth. 

The  final  product  of  the  oxidation  and  hydration  of  iron 
minerals  is  limonite,  and  the  other  oxides  of  iron  on  thorough 
alteration  yield  this  mineral.  Magnetite  in  an  outcrop  is 
commonly  present  as  an  unaltered  primary  mineral,  and  may 
not  be  taken  to  represent  the  alteration  product  of  a  sulphide 
mineralization.  Specularite,  of  characteristic  crystal  form,  is 
likewise  a  primary  mineral,  and  not  the  result  of  the  alteration 
of  sulphides:  micaceous  hematite  should  be  distinguished  from 
specularite,  as  it  is  frequently  found  as  the  alteration  product  of 
sulphides,  the  micaceous  structure  having  been  developed  by 
stress  exerted  after  its  formation.  Certain  gangue  minerals, 
such  as  lime-iron  garnet,  yield  limonite  upon  oxidation;  in  most 
cases  limonite  of  this  origin  occurs  as  soft  earthy  masses,  mixed 
with  unaltered  contact  minerals  as,  for  example,  partially 
decomposed  epidote,  and  is  commonly  distinguishable  from 
limonite  resulting  from  the  oxidation  of  sulphides. 

In  many  outcrops  in  arid  regions  whose  underlying  ore  de- 
posits have  been  explored,  the  limonite  resulting  from  the 
oxidation  of  pyrite  occurs  as  a  massive  brown  mineral,  while  the 
chalcopyrite  of  the  original  ore  is  represented  by  seamlets  and 
patches  of  soft,  bright  red  hematite. 

The  Condition  of  Outcrops  Indicative  of  Secondary  Enrichments 
in  Depth. — Features  of  outcrops  that  indicate  the  possibility  of 
enrichments  in  depth  are  the  residual  indications  of  a  good 
primary  mineralization  together  with  a  porous  or  brecciated 
structure,  or  the  presence  of  post-mineral  fractures. 


OUTCROPS  203 

A  majority  of  enrichments  of  secondary  sulphides  are  prob- 
ably due  to  the  migration  of  sulphate  and  sulphuric  acid  solutions; 
these  solutions  frequently  leave  traces  through  the  presence  of  a 
kaolinitic  alteration  of  the  associated  rocks,  or  of  kaolin  in  the  out- 
crop or  oxidized  zone.  Resistant  minerals  that  remain  after  a 
thorough  alteration  of  this  kind  are  kaolin,  limonite  and  quartz. 
Feldspars  are  completely  altered  and  sulphides  are  absent.  The 
presence  of  unaltered  feldspars  or  of  most  of  the  other  usual 
gangue  minerals  except  quartz  indicates  a  partial  alteration  at 
best,  and  the  presence  of  sulphides  indicates  an  incomplete  solu- 
tion. Galena  is  a  resistant  mineral  as  compared  with  other  sul- 
phides, and  not  infrequently  overlies  important  enrichments  of 
other  metals,  but  the  presence  of  unaltered  pyrite,  chalcopyrite 
or  zincblende  renders  it  unlikely  that  important  enrichments  will 
be  foun$  through  deeper  exploration. 

The  outcrops  of  suspected  disseminated  chalcocite  enrichments 
should'  contain  little  else  than  kaolin,  limonite  and  quartz, 
together  with,  perhaps,  some  unaltered  sericite,  itself  a  product 
of  primary  altering  agencies.  The  presence  of  pyrite  in  such  an 
outcrop  or  in  the  oxidized  zone  is  a  most  unfavorable  sign;  cases 
are  known  where  pyrite  was  found  above  secondary  chalcocite 
enrichments,  but  in  every  case  it  was  partly  altered,  crumbly, 
and  was  contained  within  thoroughly  kaolinized  rock,  the 
supposition  being  that  the  copper  was  leached  from  the  pyrite 
under  conditions  that  did  not  permit  a  complete  oxidation  of  the 
sulphur.  Where  the  containing  rock  shows  the  outlines  of  the 
feldspars  the  kaolinization  must  be  considered  unsatisfactory. 

Efflorescences  of  soluble  salts  in  the  outcrops  or  at  slight 
depth  below  them  are  frequently  instructive.  An  efflorescence 
of  copper  sulphate  is  indicative  of  the  solution  of  chalcocite,  and 
may  or  may  not  be  mixed  with  iron  sulphate.  An  efflorescence 
of  iron  sulphate  with  but  little  copper  is  commonly  indicative  of 
pyrite  at  no  great  depth.  Sulphur  and  a  yellowish-green  sul- 
phate of  iron  are  probably  not  formed  except  close  to  oxidizing 
pyritic  sulphides.  Other  efflorescences  frequently  found  are  alum 
and  zinc  sulphate,  the  latter  characteristic  of  primary  ores  below 


f 

204  EXAMINATION  OF  PROSPECTS 

any  zone  of  chalcocite  enrichment;  complex  sulphates  of  alumi- 
num and  other  bases  are  also  formed  in  the  leached  zone. 
v  Where  large  quantities  of  pyritic  sulphides  have  oxidized  and 
have  in  part  been  removed  in  solution  it  is  not  uncommon  to 
find  the  conglomerates  of  stream  beds  cemented  by  limonite, 
and,  in  some  instances,  carrying  oxidized  copper  minerals;  the 
presence  of  such  conglomerates  may  be  taken  to  indicate  con- 
ditions under  which  migration  and  enrichment  may  have  taken 
place  within  the  deposits  themselves. 

The  elevation  of  an  outcrop  as  compared  with  the  drainage 
level,  or  the  elevations  of  the  known  enrichments  of  the  district, 
are  important  factors  in  the  consideration  of  possible  secondary 
enrichments. 

-  Disseminated  chalcocite  enrichments  appear  to  be  confined  to 
regions  of  slight  rainfall:  important  enrichments  of  copper  and 
other  metals  are  found  in  veins  that  have  suffered  post-mineral 
fracturing  under  all  climatic  conditions,  except  where  rapid  ero- 
sion or  glaciation  continuously  exposes  fresh  surfaces  of  primary 
ores  and  thus  does  not  permit  the  operation  of  surface  agencies. 

Rock  Alteration  as  a  Guide  to  Ore -Deposits. — The  several 
types  of  rock  alteration  are  discussed  in  a  preceding  chapter 
where  their  close  relationship  with  mineralizing  processes  is 
emphasized.  In  the  field  it  is  usually  possible  to  distinguish 
between  a  primary  or  hydrothermal  alteration  of  the  rocks  and 
the  results  of  ordinary  weathering,  and  also  between  either  of 
these  and  kaolinization,  but  where  such  distinction  is  doubtful, 
slides  should  be  cut  and  examined  under  the  microscope,  when 
the  type  of  alteration  will  become  apparent.  A  "  highly  altered 
condition"  means  nothing  unless  its  type  and  probable  relation 
to  mineralization  are  understood. 

The  Outcrops  of  Kaolinized  Rocks. — Upon  the  kaolinitic  altera- 
tion of  rocks  carrying  pyritic  mineralizations  the  products  of 
the  alteration  are  likely  to  be  similar  whatever  the  original 
nature  of  the  individual  rocks.  On  Shannon  Mountain,  Arizona, 
granite,  porphyry,  and  shales  have  all  suffered  intense  kaolinitic 
alteration,  and  the  resulting  mass  of  kaolin,  sericite,  and  quartz 


OUTCROPS  205 

with  associated  limonite  and  chalcocite  may  rarely  be  differen- 
tiated in  the  field  into  parts  representing  the  original  types  of 
rocks.  In  granitic  rocks  that  were  not  sericitized  before 
suffering  kaolinization  the  quartz  phenocrysts  remain  clearly 
distinguishable,  and  in  hand  specimens  tend  to  obscure  the 
degree  of  alteration;  a  kaolinitic  alteration,  however,  that  has 
not  obliterated  the  outlines  of  the  feldspars  cannot  be  considered 
thorough,  and  outcrops  of  this  nature  are  rarely  underlain  by 
important  enrichments.  Thorough  leaching  and  kaolinization 
usually  removes  the  iron  as  well  as  the  other  bases,  and  the 
dumps  of  workings  in  the  leached  zones  are  commonly  white 
in  color,  in  sharp  contrast  with  the  brown  or  red  color  of  the 
surface.  In  deposits  that  originally  carried  abundant  pyrite, 
much  limonite  may  remain  in  the  leached  zone.  The  line  of 
demarkation  between  the  leached  and  the  enriched  zones  in 
such  cases  is  well  marked,  the  former  being  stained  by  iron,  while 
the  latter  is  white  in  color. 

The  Outcrops  of  Contact  Deposits. — The  minerals  developed 
by  contact  metamorphism  are,  in  general,  resistant  to  oxidation 
and  erosion,  and  are  likely,  therefore,  to  form  conspicuous  out- 
crops. The  tightness  of  these  minerals  and  the  slowness  with 
which  they  decompose  under  the  influence  of  surface  agencies 
protect  the  sulphides  contained  within  them  and  so  in  large 
measure  prevent  migration  and  enrichment.  A  further  hinder- 
ance  to  secondary  enrichment  in  deposits  of  this  nature  is  the 
usual  presence  of  active  precipitants.  As  a  result  of  these  con- 
ditions secondary  enrichments  are  rare  in  contact  deposits  ex- 
cept where  the  sulphides  were  present  in  large  masses;  in  all 
other  cases  the  outcrops  of  contact  deposits  are  likely  to  be  in- 
dicative of  the  values  contained  by  the  deposit  at  all  depths. 

Deposits  of  Surface  Origin. — Certain  deposits  of  surface 
origin  present  close  imitations  of  the  outcrops  of  ore-deposits, 
but  are  not  underlain  by  valuable  minerals.  Bog  iron  ore 
(limonite)  is  a  familiar  example  of  surficial  deposit  not  connected 
with  any  underlying  mineralization,  and  many  deposits  of  the 
oxides  of  manganese  also  occur  in  like  manner.  Nodular  con- 


206 


EXAMINATION  OF  PROSPECTS 


cretions  of  manganese  oxides  that  form  on  the  sea  bottom  are 
likely  to  be  concentrated  into  such  superficial  deposits,  as  is 
also,  upon  erosion,  the  manganese  contained  in  small  quantities 
by  many  rocks.  Bog  iron  ores  commonly  contain  casts  of  vege- 
table remains,  sand,  and  silt,  and  may  also  be  recognized  by 
their  bedded  form.  Surficial  deposits  of  limonite  and  manganese 
oxides  may  generally  be  distinguished  from  the  outcrops  of 
mineral  deposits  through  their  lack  of  associated  minerals  char- 
acteristic of  outcrops.  Furthermore,  after  slight  exploration 
their  structure  is  revealed  and  their  superficial  nature  becomes 
apparent.  Pyritic  deposits  are  occasionally  met  with  where  the 
sulphides  have  replaced  roots,  or  other  organic  matter,  and 
whose  superficial  formation  is  evident. 


FIG.  78. — Section  of  open  cut,  Bertha  Zinc  Mines,  Virginia,  showing  the 
superficial  position   of  the  residual   ore.     After  Watson. 

Microscopic  Examination  of  Specimens. — The  investigation  of 
an  outcrop  is  largely  a  search  for  residual  conditions  indicating 
that  an  important  mineralization  has  been  removed  from  the 
surface  by  oxidation  and  solution.  It  is  often  advisable  to  have 
slides  cut  from  specimens  of  an  outcrop  and  its  associated  rocks, 
and  to  have  them  examined  under  the  microscope.  Information 
is  thus  gained  of  the  character  and  degree  of  the  alteration,  and 
in  many  cases  of  the  mineralogical  nature  of  the  original  ore  and 
of  the  distribution  of  the  several  minerals.  Specimens  taken  for 
this  purpose  should  be  chosen  carefully,  and  a  record  of  them 
kept  in  the  same  way  as  is  the  custom  with  samples  taken  for 
assay:  furthermore,  it  is  best  always  to  reserve  a  specimen, 


OUTCROPS 


207 


preferably  part  of  the  same  piece  that  is  sent  away,  for  purposes 
of  comparison  upon  receipt  of  the  results  of  the  examination. 
Microscopic  examination  is  of  great  value,  if  only  for  corrobo- 
ration  of  the  evidence  gathered  in  the  field,  but  may  also  yield 
information  and  indicate  possibilities  that  without  it  would  not 
be  suspected. 


10 


zo  Feet 


FIG.  79. — Example  of  surficial  deposit;  nodules  of  manganese  oxide.     After 

Harder. 


DESCRIPTIONS  OF  OUTCROPS 

v  AT  NACOZARI,  SONORA,  MEXICO,  the  outcrops  of  the  Pilares 
solution-breccia  are  prominent.  The  country  rocks  of  this  dis- 
trict are  andesitic  and  rhyolitic  porphyries  whose  surface  over  an 
area  of  several  square  miles  is  stained  red,  yellow  or  brown  by 
the  oxides  of  iron  resulting  from  the  oxidation  of  a  disseminated 
cupriferous  pyritic  mineralization.  Shallow  leaching  and  attend- 
ant unimportant  enrichments  of  disseminated  chalcocite  and 
chalcopyrite  are  present  at  many  points  in  the  district,  but  the 
depth  and  degree  of  alteration  and  enrichment  are  apparently  too 


208  EXAMINATION  OF  PROSPECTS 

slight  to  yield  ore-bodies  of  commercial  importance.  The  walls 
of  the  gulches  frequently  show  efflorescences  of  chalcanthite, 
usually  associated  with  ferric  sulphate  and  sulphur,  the  ear  marks 
of  a  near-by  unaltered  pyritic  mineralization:  a  white,  probably 
aluminous,  precipitate  is  present  at  many  points  where  solutions 
are  oozing  out  of  the  rocks,  and  this,  apparently,  is  also  a  sign  of 
shallow  alteration.  In  the  vicinity  of  the  Pilares  ore-body,  the 
only  important  copper  deposit  in  the  district,  the  rock  is  strongly 
sericitized,  and  in  this  respect  differs  from  the  remainder  of  the 
district  seen  by  the  writer.  The  outcrop  of  this  ore-deposit  is 
composed  of  a  striped  breccia,  seamlets  of  ore  minerals  being 
separated  by  long,  thin  slabs  or  sharp  angular  fragments  of  the 
country  rock  that  frequently  show  a  curving  parallel  arrange- 
ment of  ore-  and  rock-areas.1 

The  chief  ore  mineral  is  chalcopyrite,  with  some  bornite,  as- 
sociated with  pyrite  and  quartz.  In  the  outcrop,  the  seamlets 
residual  after  the  ore  are  made  up  principally  of  micaceous 
hematite  and  quartz,  with  small  particles  of  copper  carbonates. 
Through  the  brown  micaceous  hematite,  and  as  individual  seams, 
occurs  bright  red  hematite,  in  distribution  following  that  of  the 
chalcopyrite  in  the  ore  of  the  sulphide  zone,  and  evidently 
residual  after  this  mineral.  The  outcrop  is  of  small  extent  as 
compared  with  that  of  the  deposit  as  developed  in  the  deep 
levels. 

V  AT  JEROME,  ARIZONA,  the  conditions  exposed  in  the  open  pit  of 
the  United  Verde  mine  are  indicative  of  intense  mineralization. 
Here  the  principal  country  rocks  appear  to  be  schist  and  a 

1  This  structure  is  considered  by  Mr.  S.  F.  Emmons  to  indicate  minerali- 
zation attended  by  a  splitting  off  of  the  country  rock  in  concentric  shells 
under  the  influence  of  the  mineralizing  solutions;  this  type  of  deposit  is 
termed  by  him  a  "solution  breccia."  The  writer  has  seen  the  same  type 
of  mineralization  at  several  other  districts,  all  of  them  in  Sonora;  in  every 
case  deposits  were  contained  in  deep  flows,  or  sills,  of  andesitic  or  rhyolitic 
porphyry.  At  San  Antonio  de  la  Huerta  the  copper  deposits  are  of  this 
type;  at  La  Trinidad,  Sahuaripa,  the  lead  silver  deposits  occur  in  a  solution 
breccia,  as  do  also  the  silver  deposits  at  Guadalupe;  the  gold  deposits  near 
Bacoachi  are  said  to  be  solution  breccias. 


OUTCROPS  209 

rather  basic  granite-porphyry  that  carries  a  heavy  pyritic 
mineralization.  The  residual  minerals  remaining  in  the  walls 
of  the  pit  from  which  the  surface  ores  were  mined  are  white 
kaolin,  quartz,  limonite,  and  oxides  of  manganese  in  which 
occurs  copper  carbonates  and  silicates.  The  outcrop  is  said  to 
have  carried  gold  values.  Sharp  lines  of  demarkation  exist 
between  bands  of  unaltered  schist  and  the  mineralized  and 
altered  parts  of  the  deposit.  Kaolinization  does  not  appear 
to  have  affected  the  wall  rocks.  The  principal  ore  minerals  in 
depth  are  auriferous  chalcopyrite  and  bornite. 

AT  MORENCI  AND  METCALF,  ARIZONA,  the  lode  and  the  dissemi- 
nated deposits  in  porphyry  have  inconspicuous  outcrops.  The 
lodes  are  represented  at  the  surface  by  quartz-filled  fractures 
through  sericitized  and  kaolinized  porphyry.  A  light  iron  stain 
is  common  at  the  surface  throughout  the  intrusive  areas,  but, 
in  general,  the  iron  as  well  as  the  copper  has  been  removed 
from  the  mass  of  the  leached  zone  by  solution.  Except  where 
the  secondary  ores  actually  outcropped,  as  is  said  to  have  been 
the  case  with  the  Metcalf  ore-bodies,  there  is  but  little  indica- 
tion of  copper  at  the  surface,  although  a  little  chrysocolla  is 
occasionally  seen,  and  light  efflorescenses  of  chalcanthite  are 
common  on  the  walls  of  shallow  workings  driven  into  the  por- 
phyry. The  most  promising  areas  for  disseminated  chalcocite 
enrichments  are  indicated  by  abundant  casts,  residual  after  an 
intense  primary  pyritic  mineralization,  together  with  ramifying 
quartz  veinlets  associated  with  a  thorough  kaolinization  of  the 
rock.  All  of  the  important  enrichments  in  this  district  occur 
high  up  in  the  hills;  the  canyons  are  in  every  case  in  lean  pri- 
mary ore,  and  extensive  exploration  of  the  lower  hill  slopes  has 
been  without  result. 

AT  BISBEE,  ARIZONA/  one  only  of  the  large  pyritic  ore  bodies 
outcropped  at  the  surface;  in  this  deposit  the  usual  oxidized 
copper  minerals  were  present  in  quantities  sufficient  to  con- 
stitute ore.  The  limestone  in  the  vicinity  of  many  of  the  ore- 
bodies  is  fractured  and  carries  disseminated  pyrite.  The  rusty 

1  F.  L.  Ransome,  P.  P.  21,  U.  S.  G.  S., 

14 


210  EXAMINATION  OF  PROSPECTS 

outcrops  of  this  pyritic  mineralization  have  in  some  instances 
indicated  the  presence  of  underlying  ore-bodies;  slight  depres- 
sions were  noted  at  the  surface  above  certain  ore-bodies,  probably 
due  to  subsidence  upon  the  removal  in  solution  of  certain 
constituents  of  the  deposits.  Many  of  the  most  important  of 
the  ore-bodies  of  this  distinct,  however,  are  not  represented  in 
any  way  at  the  surface.  The  intensely  mineralized  porphyry 
of  Sacramento  Hill  taken  in  connection  with  the  fracturing, 
silicification  and  marmorization  of  the  limestone  are  surface 
conditions  patently  favorable  to  the  existence  of  ore-deposits. 

AT  BINGHAM,  UTAH/  the  outcrops  of  the  lead-silver,  and  the 
copper  deposits  of  the  Old  Jordan  group  are  inconspicuous. 
The  containing  limestone  is  irregularly  altered  to  white  or  gray 
marble,  often  carrying  structureless  patches  of  decomposed 
powdery  limestone;  silicification  is  extensive,  quartz  occurring 
as  granular  masses,  as  fine  compact  replacements,  and  as  cherty 
bands  and  patches.  The  outcrops  of  the  lodes,  which  are  trace- 
able with  difficulty,  are  belts  of  shattered  and  discolored  rock; 
the  fissure  fillings  are  marked  by  crushing  and  are  stained  by 
the  oxides  of  iron  and  copper  carbonates.  The  replacement 
deposits  are  represented  at  the  surface  by  shattering,  silicification, 
and  discoloration  of  the  limestone.  At  many  points  the  rusty, 
siliceous  outcrops  carried  gold  values.  An  upper  zone  of  car- 
bonates and  oxidized  ores  here  is  commonly  underlain  by 
enriched  sulphides. 

AT  PARK  CITY,  UTAH,  the  ores  occur  as  lode  and  replacement 
deposits  in  limestone;  outcrops  are  scanty  or  lacking.  Marmor- 
ization and  silicification  of  the  limestone  are  present  in  places, 
and  shattering  and  discolorization  are  the  chief  surface  indica- 
tions of  the  important  deposits. 

IN  THE  COEUR  D'ALENE  DISTRICT,  IDAHO,*  the  outcrops  of  the 
important  lead-silver  deposits  in  limestone  are  inconspicuous. 
Soil  and  vegetation  cover  the  hillsides,  and  the  material  of  the 
lodes  is  neither  so  superior  to  the  enclosing  rock  in  hardness  or 

1  J.  M.  Boutwell,  P.  P.  38,  U.  S.  G.  S.,  p.  238. 

2  F.  L.  Ransome,  P.  P.  62,  U.  S.  G.  S.,  p.  129. 


OUTCROPS  211 

durability  as  to  form  bold  outcrops,  nor  so  easily  eroded  as  to 
produce  trenches  or  saddles  in  the  topography.  The  courses  of 
the  lodes  may  not  be  followed  at  the  surface  without  the  aid  of 
test  pits  or  trenches.  The  ores  at  the  surface  carry  galena  with 
a  little  cerussite,  limonite  and  copper  carbonates. 

AT  METCALF,  ARIZONA  ,  large  contact  metamorphic  deposits 
form  the  summit  of  Shannon  Mountain,  their  resistant  minerals 
having  halted  erosion  at  the  horizon  of  greatest  development. 
The  limestone  strata  have  been  largely  altered  by  contact  meta- 
morphism  to  magnetite  and  garnet  with  abundant  pyrite  and 
associated  chalcopyrite. 

Oxidation  has  worked  over  these  deposits  thoroughly;  while 
part  of  the  copper  has  been  removed  by  solution,  in  the  sedi- 
ments most  of  the  copper  originally  present  probably  remains  as 
residual  ores;  the  magnetite  and  garnet  have  been  partly  altered 
to  limonite  and  quartz.  The  primary  pyritic  mineralization 
was  sufficiently  intense  to  have  permitted  access  by  surface 
waters  and  thorough  oxidation.  The  residual  ores  are  chiefly 
azurite,  malachite,  chrysocolla,  and  brochantite,  associated  with 
limonite,  hematite,  and  the  oxides  of  manganese.  In  the  por- 
phyry areas  the  surface  is  kaolinized  and  leached,  and  underlying 
enrichments  of  chalcocite  are  found  in  large  deposits,  both  as 
disseminated  enrichments,  and  in  lode-like  deposits  along  fissures. 
Near  the  top  of  the  enriched  zone  further  oxidation  has  produced 
cuprite  and  some  native  copper  from  the  chalcocite.  The  shales, 
in  part  altered  to  hornfels  by  contact  metamorphism,  received 
in  places  a  disseminated  mineralization  of  pyrite  and  chalcopyrite 
which  have  probably  been  somewhat  enriched  by  circulating 
solutions. 

»  AT  SAN  PEDRO,  NEW  MEXICO/  copper  occurs  as  chalcopyrite 
associated  with  garnet  in  extensive  contact  deposits.  These 
deposits  are  confined  to  the  lower  part  of  a  laccolithic  roof  of 
limestones.  The  shaly  limestone  is  in  places  altered  to  hornfels 
and  carries  tremolite  and  diopside  as  coarse  crystals:  this  type 
of  altered  rock  carries  no  ore;  above  these  beds  occur  garnetized 

1  Waldemar  Lindgren,  P.  P.  68,  U.  S.  G.  S.,  p.  173. 


212  EXAMINATION  OF  PROSPECTS 

beds  from  50  to  100  ft.  thick  and  of  great  horizontal,  extent, 
through  which  the  ore  is  irregularly  distributed.  The  primary 
ore  is  composed  of  chalcopyrite,  with  some  gold,  in  a  yellowish- 
garnet,  chiefly  andradite,  calcite,  and  lesser  quantities  of  tremo- 
lite  and  wollastonite.  Oxidation  has  had  but  slight  effect  on 
these  deposits,  in  which  apparently  there  has  been  no  migration 
or  enrichment. 

AT  VIRGINIA  CITY,  NEVADA,  the  Comstock  Lode  outcrops  for  a 
long  distance  as  siliceous  masses  and  quartz  veinlets  cementing 
a  fractured  zone.  This  lode,  unlike  most  deposits,  occupies  a 
fault  of  large  displacement.  The  country  rocks  are  propylitized 
andesites.  The  lode  is  continuous  over  its  central  part,  but 
branches  at  either  end;  while  broad  and  scattered  at  the  surface, 
it  becomes  more  regular  in  depth.  The  principal  values  at  the 
surface  were  as  chloride  of  silver.  The  bonanza  ores  contained 
stephanite,  polybasite,  argentite,  and  native  gold,  associated  with 
small  quantities  of  galena  and  zincblende  in  a  quartz  gangue. 
,  AT  TONOPAH,  NEVADA1  the  important  veins  showed  prominent 
and  continuous  outcrops  of  white  quartz;  the  first  samples 
broken  from  the  veins,  although  rich,  showed  no  ore  minerals, 
and  were  of  unpromising  appearance.  The  quartz  has  in  places 
a  purplish  color,  due  to  minute  particles  of  argentite.  The  ores 
near  the  surface  carried  chloride,  bromide  and  iodide  of  silver 
associated  with  limonite  and  oxides  of  manganese.  The  country 
rock  is  a  sericitized  andesite. 

;  AT  SILVERTON,  COLORADO,2  the  outcrops  of  the  stock  deposits  of 
the  Red  Mountain  area  form  prominent  siliceous  knobs  composed 
of  silicified  andesite  carrying  finely  disseminated  pyrite,  sericite 
and  kaolin.  These  knobs  are  thoroughly  fractured  and  contain 
vugs,  cavities,  and  ramifying  caves.  The  residual  ores  were 
found  chiefly  in  the  caves,  as  beds  of  sandy  or  clayey  material, 
and  on  their  walls  associated  with  porous,  spongy  masses  of 
quartz.  The  oxidized  ores  carried  argentiferous  cerussite  and 
anglesite  associated  with  siderite,  barite,  "'oxide  of  iron"  and 

1  J.  E.  Spurr,  P.  P.  42,  U.  S.  G.  S.,  p.  122. 

2  F.  L.  Ransome,  Bull.  182,  U.  S.  G.  S.,  p.  233. 


OUTCROPS  213 

kaolin.  At  slight  depth  argentiferous  galena  formed  the  principal 
ore;  this  gave  way  in  depth  to  argentiferous  enargite,  chalcocite, 
bornite  and  chalcopyrite,  which  in  turn  were  underlain  by  lean 
primary  pyritic  sulphides. 

AT  CRIPPLE  CREEK,  COLORADO,  the  telluride  gold  ores  occur  with 
a  very  scanty  gangue,  and  the  outcrops  of  the  veins  are  not 
conspicuous.  "As  elements  of  geological  structure,  the  lode 
fissures  at  Cripple  Creek  are  exceedingly  inconspicuous.  They 
are  marked  neither  by  bold  outcrops  of  quartz  nor  by  superficial 
bands  of  ferruginous  gossan.  They  seldom  fault  perceptibly 
the  structures  that  they  traverse,  and  are  not  sufficiently  different 
from  the  enclosing  rocks  as  regards  resistance  to  erosion,  to  have 
influenced  perceptibly  the  topographic  development  of  the 
district.  It  is  this  obscurity  that  retarded  the  discovery  of  the 
ore-deposits,  and  that  to-day  renders  it  impossible  to  follow  the 
veins  over  the  surface  without  first  stripping  off  the  soil  and  loose 
rock,  or  sinking  test  pits."1 

The  surface  ores  here  contain  dull  native  gold  associated  with 
tellurites  in  ferruginous  clays,  kaolin  and  alunite.  Hydrother- 
mal  metamorphism  was  an  inconspicuous  process  in  this  district. 

IN  THE  BLACK  HILLS,  SOUTH  DAKOTA,2  the  large  Homestake 
ore-bodies  outcropped  as  iron  stained  chloritic  slates,  quartz,  and 
porphyry,  which  in  places  carried  as  high  as  $16  in  gold.  These 
altered  gold-bearing  rocks  were  not-  sufficiently  resistant  to  ero- 
sion to  produce  prominent  outcrops.  In  the  open  cuts  it  is  not 
possible  for  the  unpractised  eye  to  distinguish  the  slates  that  are 
sufficiently  mineralized  to  constitute  ore  from  those  that  are 
practically  barren.  In  general,  the  rocks  within  the  ore  zone 
appear  to  be  somewhat  more  completely  silicified,  and  to  carry 
more  iron  stain  than  the  country  rocks,  and  also  to  have  been 
subject  in  greater  degree  to  fracturing  and  folding.  In  the  sur- 
face ores  the  quartz  occurs  in  three  phases:  as  thin  silicious 
layers  intercalated  in  the  slates,  as  thin  seams  that  frequently 
follow  the  lamination  and  bedding,  but  occasionally  cut  across 

1  Waldemar  Lindgren  and  S.  F.  Emmons,  P.  P.  54,  U.  S.  G.  S.,  p.  153. 

2  J.  D.  Irving,  P.  P.  26,  U.  S.  G.  S.,  p.  57. 


214  EXAMINATION  OF  PROSPECTS 

them,  and  also  as  veinlets  independent  of  the  structural  features 
of  the  containing  rock,  which  last  are  the  most  important.  The 
gold  in  these  deposits  occurs  in  finely  disseminated  particles,  and 
appears  to  be  of  later  origin  than  the  quartz  and  rock  gangue. 

IN  THE  MOGOLLONES  DISTRICT,  NEW  MEXICO/  the  country  rocks 
are  a  series  of  flows,  fragmental  beds  and  tuffs,  composed  of  soda- 
rhyolite,  andesite  and  basalt.  The  rocks  in  the  mineralized  areas 
exhibit  propylitic  alteration;  silicification,  resulting  in  a  green- 
ish-gray hornstone,  is  prominent  along  the  veins.  Sericitic  alter- 
ation is  absent.  The  veins  are  partly  filled  fissures  and  partly 
the  result  of  replacement  of  the  walls  along  fractured  zones.  The 
topography  is  rugged,  and  the  veins  being  harder  than  the  altered 
wall  rocks,  form  bold  outcrops.  The  veins  carry  native  silver, 
argentite,  chalcocite,  pyrite,  chalcopyrite,  and  bornite  associated 
with  specularite  in  a  gangue  of  quartz,  calcite  and  fluorite.  In 
certain  of  the  veins  in  which  copper  is  present  in  small  quantity 
the  ore  minerals  are  finely  disseminated  through  the  gangue.  In 
another  type  of  vein  copper  forms  a  large  proportion  of  the  total 
value  of  the  ore  and  sulphides  are  abundant.  Oxidation  has 
reached  a  slight  depth  only  in  these  veins  and  secondary  enrich- 
ment does  not  appear^  to  have  effected  any  important  rearrange- 
ment of  the  values.  The  water  level  in  this  district  is  deep,  and 
the  slight  depth  of  secondary  alteration  and  oxidation  must  be 
explained  by  the  geologic  youth  of  the  region,  erosion  having 
proceeded  more  rapidly  than  oxidation.  The  degree  to  which  the 
rocks  have  been  shattered  and  the  presence  of  abundant  druse- 
lined  cavities  in  the  veins,  together  with  the  type  of  rock  altera- 
tion, indicate  a  slight  depth  at  the  time  of  ore  deposition,  and 
that  no  great  depth  of  rock  has  been  removed  by  erosion  since 
the  veins  were  formed.  The  oxidized  ores  found  at  the  surface 
contained  chiefly  malachite,  cerargyrite,  and  gold,  associated  with 
limonite. 

1  L.  C.  Graton,  P.  P.  68,  U.  S.  G.  S.,  p.  195. 


INDEX 


Adirondack  Mts.,  deposits  of,  87 
Aereal  geology,  40 
Aguilera,  J.  G.,  80 
Alaska-TreadweU  mine,  7,  47,  77 
Alteration,  alunitic,  150 

contact,  146 

decomposition,  153 

depth  of,  167 

dynamo-regional,  145 

to  greisen,  150 
.  hydrometamorphic,  153 

hydrothermal,  146 

influence  of  channels  on,  160 

kaolinitic,  155 

leaching,  154 

metasomatic,  82,  84 

migration,  158 

oxidation,  156 

primary,  145,  204 

propylitic,  148 

secondary,  153 

sericitic,  149 

silicification,  150 

by  surface  agencies,  153 

of  wall  rocks,  145 

weathering,  153 
Alunitic  alteration,  150 
Anticlines,  31 

ore-deposits  at  tops  of,  134 
Antiguas,  examination  of,  4 
Apatite  veins,  83 
Apex,  7 

Appalachians,  deposits  of,  71 
Aspen,  Colo.,  126,  141 
Associations,  of  metals,  160 

of  minerals,  76 


Bain,  H.  F.,  193 

Bald  Mountain,  S.  D.,  59,  106 

Ballerat,  Australia,  141 

Basin,  31 

Bassick  mine,  Colo.,  54 

Bayley,  W.  S.,  30 

Beck,  R.,  32,  33,  53,  55,  63,  79,  80, 

109,  110,  112,  128,  129,  185 
Bed  vein,  45 
Bedded  ore-deposits,  110 
Bendigo,  Australia,  124,  135 
Bertha  mines,  Va.,  206 
Bibliography,  search  of,  6 
Bingham,  Utah,  58,  108,  126,  184, 

•210 

Bisbee,  Ariz.,  79,  150,  181,  209 
Black  Hills,  S.  D.,  32,  103,  106,  129, 

132,  213 

Blanket  vein,  45 
Boehmer,  Max,  160 
Booms,  mining,  3 
Boutwell,  J.  M.,   58,    80,    105,    108, 

126,  184,  210 

Branching  veins,  54,  61,  81 
Breccia  lodes,  52 
Broken  Hill,  N.  S.  W.,  189 
Bullfrog,  Nev.,  198 
Bunker  Hill  mine,  Ida.,  104 
Burro  Mts.,  N.  M.,  108 
Butte,  Mont.,  118,  140 

Carbon,  precipitation  by,  138 
Casts  in  resistant  minerals,  200 
Cassiterite  veins,  83 
Catalina  Mts.,  deposits  of,  7 
Cerro  de  Potosi,  S.  A.,  59 


215 


216 


INDEX 


Chalcanthite,  9,  15 
Chalcocite,  enrichments,  ISO 

disseminated,  182 
Chalcopyritic  enrichments,  184 
Chimneys,  53 
Church,  J.  A.,  133,  134 
Clark,  F.  W.,  195 
Classification  of  ore-deposits,  86 
Cleavage,  26 

Clifton,  Ariz.,  78,  108,  179,  181 
Clinton  ore  measures,  111 
Coeur  d'Alenes,   Ida.,   46,   80,    104, 

117,  124,  210 
Compensating  faults,  35 
Comstock  lode,  Nev.,  212 
Compound  veins,  46,  47 
Concentrations,  surface,  171 
Concretions,  oolitic,  111 
Conglomerate  beds,  109 
Conjugate  faults,  35 

joints,  56 

veins,  56,  57 
Contact  deposits,  87,  88,  157 

metamorphism,  146,  152 

ores,  79,  124 

veins,  47 
Contouring,  40 
Copper,  enrichment  of,  180 

oxidized  ores  of,  169 

primary  ores  of,  78,  79 

secondary  ores  of,  176 
Copper  King  mine,  Ariz.,  181 
Cordilleras,  deposits  of,  71,  72 
Cornwall,  England,  137,  138 
Cost  of  mining,  22 
Cripple  Creek,  Colo.,  48,  49,  59,  60, 

77,  119,  129,  213 
Crosscutting,  9,  42,  49,  57 
Crosscut  tunnels,  9 
Crustification,   39,    74,    94,    96,    97, 

,136 

Crystallization,  interrupted,  77,  200 
Curie,  J.  H.,  5,  24,  141 


Darton,  N.  H.,  29 
Decomposition  of  rocks,  153 
Deep-seated  veins,  83 
Deformation,  37 
De  la  Beche,  138 
De  la  Mar  mine,  Utah,  171 
Development,  40 
Devil's  Tower,  Wyo.,  29 
Diller,  J.  S.,  78 
Displacement,  33,  34,  37 
Disseminated  enrichments,  182 

mineralizations,  107 
Dolcoath  mine,  England,  137 
Dolomitization,  152 
Dome,  31 
Drag,  36 

Dressing  a  mine,  10 
Ducktown,  Tenn.,  78,  91,  181 
Dunnington,  F.  P.,  195 

Efflorescent  salts,  15,  159,  203 
Emmons,    S.  F.,   35,   80,    171,    189, 

215 
Emmons,  W.  H.,  75,  79,  91,  92,  122, 

164,  165,  181,  191 
Encampment,  Wyo.,  30,  56 
Enrichment,  chalcocitic,  180 

chalcopyritic,  184 

of  copper,  180 

depths  of,  163,  165 

disseminated,  182 

of  gold,  189 

of  gold  and  silver  with  copper, 
185 

irregularity  of,  162 

of  lead,  191 

of  lesser  metals,  195 

of  metals,  158 

relation  of,  to  topography,  161 

relation  of,  to  water  level,  161 

of  silver,  186 

of  zinc,  194 

zones  of,  158,  163 


INDEX 


217 


Epigenetic  deposits,  26 
Examinations,  of  antiguas,  4 

formal,  1 

preliminary,  1 

preparing  a  property  for,  9 

of  prospects,  3 

for  rescue  of  capital,  2 
Exploration,  71,  139 

amount     of,     compared     with 
results  attained,  9 

and  development,  40 

for  faulted  ore-body,  36 

of  prospects,  6 
Extrusive  rocks,  69 

Fahlbands,  90 
Falun,  Sweden,  79 
Faribault,  E.  R.,  135 
Faults,  33 

blocks,  40 

compensating,  35 

depth  of  formation,  35,  36 

development  from  folds,  31 

expression  in   topography,    39, 
199 

lodes,  51 

normal,  34 

reverse,  34 

step,  35 

torsion,  34 
Filled  fissures,  39,  74,  93,  95,  96,  97, 

136 

Filling  of  open  spaces,  93 
Finlay,  J.  R.,  22 
Fissility,  27 

Fissures,  gouge  filled,  39 
Fissuring,  time  of,  68 
Flexures,  32,  33 
Folds,  31 

Formal  examinations,  1 
Foster,  Le  Neve,  137 
Fractures,  32 
Freiberg,  Germany,  63 


Gale,  H.  S.,  30 

Gallup,  N.  M.,  31 

Garrey,  G.  H.,  64,  66,  187,  188,  192 

Gash  veins,  44 

Gemmel,  R.  C.,  13 

Geological  maps,  3,  4,  8,  37,  40 

sections,  37,  40 
Geology,  aereal,  40 

structural,  26 
Georgetown,  Colo.,  64,  80,  130,  187, 

192 

Gilbert,  G.  K.,  70 
Globe,  Ariz.,  199 
Gneissic  structure,  27,  67 
Gold,  enrichment  of,  185 
distribution  of  by     • 
oxidation,  171 

primary  ores  of,  77,  78 
Goldfield,  Nev.,  43,  151 
Gordon,  C.  H.,  90 
Gossan,  202 
Gouge  filled  fissures,  39 
Granby  mine,  B.  C.,  8 
Granite  mine,  Mont.,  79,  164,  191 
Grant  Co.,  N.  M.,  133 
Grant,  U.  S.,  193 
Grass  Valley,  Cal.,  78,  123 
Graton,  L.  C.,  90,  105,  109,  214 
Guanajuato,  Mex.,  80 
Gympie,  Australia,  141 

Hand  picking,  2,  21 
Hanging  wall,    relative   mineraliza- 
tion of,  85 
strength  of,  8 
Harder,  E.  C.,  207 
Harz,  Germany,  55 
Heave,  33 

Helena-Frisco  mine,  Ida.,  117 
Hermosa,  N.  M.,  133 
Herron,  J.,  56 
High  assays,  19 
Highland  Boy  mine,  Utah,  105,  184 


218 


INDEX 


Himmelsfahrt,  Germany,  128 
Homestake  mine,  S.  D.,  213 
Horn  Solver,  Utah,  171 
Howell,  E.  E.,  31 
Hundt,  R.,  31 
Hydrometamorphism,  1531 
Hydrothermal  metamorphism,  146 

alunitic,  150 

distinction  from  weathering,  1 48 

to  greisen,  150 

propylitic,  148 

sericitic,  149 

silicification,  150 

Idaho  Springs,  Colo.,  66 

Inclusions,  fluid,  74 

Indicator  seam  at  Ballerat,  142 

Intercalated  veins,  67 

Intergrowth,  structural,  74 

Intrusive  rocks,  relation  of  minerali- 
zation to,  69,  70 

Irving,  J.  D.,  32,  35,  79,  105,  129, 
132,  188,  213 

Jerome,  Ariz.,  208 
Joints,  27 

compression,  27 

conjugate,  56 

mineralization  of,  51,  56 

tension,  28,  31 
Jones  sampler,  13 

Kaolin,  precipitation  by,  168 

Kaolinization,  154,  155 

Kemp,   J.  F.,   34,   44,  87,  90,   110, 

111 

Keweenaw  Point,  Mich.,  110 
Kingston,  N.  M.,  133 
Krumb,  H.,  184 

Laccoliths,  70 

Lake  City,  Colo.,  79,  188 

Lake  Valley,  N.  M.,  133 


Laney,  F.  B.,  91,  181 
Large  vs.  small  properties,  11 
Leaching,  154,  159 
Leaching  (process),  9 
Lead,  enrichment  of,  191 

oxidized  ores  of,  169 

primary  ores  of,  80 
Leadville,  Colo.,  35,  80,  160,  195 
Ledges,  44 
Lincoln,  F.  O.,  187 
Lindgren,  W.,  44,  49,  71,  75,  77,  78, 
79,  82,  86,  88,  89,  90,  92, 
108, 116, 119, 123,  127,  129, 
145,  146,  157, 158,  160,  176, 
179,  184,  195,  211,  213 
Linked  veins,  55 
Local    data,    better    than    general 

data,  37,  114,  138 
Lodes,  44 

breccia,  52 

fault,  51 

sheeted,  48 

stringer,  50 

Losses,  metallurgical,  21 
Low  grade  ores,  11 

McCaffery,  R.  S.,  79 

Magmatic  segregations,  86 

Malcolmson,  J.  W.,  102 

Manganese,  distribution  of,  by  oxi- 
dation, 172 

Mansfeld,  Germany,  112 

Marmorization,  152 

Meandering  of  outcrops,  197 

Metals,  associations  of,  123,  160 
migration  of,  158 
vertical  sequence  of,  123 

Metallogenetic  epochs  and  provinces, 
71 

Metallurgical  losses,  8,  21 

Metamorphism,  contact,  146,  152 
dynamo-regional,  91,  146 
hydrothermal,  146,  148.  150 


INDEX 


219 


Metasomatic  alteration,  see  altera- 
tion, metasomatic. 

replacement,  99 

Metcalf,  Ariz.,  108,  177,  209,  211 
Miami,  Ariz.,  184 
Michigan,  copper  deposits  of,  110 
Microscopic  slides,  73,  206 
Migration  of  metals,  158 

of  gangue  minerals,  195 

upward,  160 
Mill  runs,  17 

Mine  dressed  for  examination,  10 
Minerals,  accessory,  76 

both  primary  and  secondary,  75 

primary,  74,  75 

secondary,  76 
Mining,  cost  of,  22 
Mining  examinations,  1 

formal,  1 

for  rescue  of  capital,  2 

preliminary,  1 

of  prospects,  3 
Mogollones,  N.  M.,  214 
Monocline,  31 

Monte  Cristo,  Wash.,  51,  191 
Montpelier,  Ida.,  30 
Monzonite,  association  of  ores  with, 

72 

Morenci,   Ariz.,   89,    108,    174,    177, 
184,  209 

Nacozari,  Mex.,  8,  207 

Negaunee,  Mich.,  30 

Neihart,  Mont.,  65,  67,  140,  189 

Nieggen,  Germany,  31 

Nogal,  N.  M.,  109 

Normal  faults,  34 

Nova  Scotia,  gold  deposits  of,  135 

Offset,  33 

Old  Jordan  mine,  Utah,  210 
Oolitic  concretions,  111 
Open  spaces,  95 


Ordonez,  E.,  80 
Ore  deposition,  loci  of,  69,  70 
Ore-deposits,    associated    with   cer- 
tain rocks,  71 
behavior  of  in  depth,  81 
deep-seated,  81 
distribution  in  districts,  71 
epigenetic,  26 

formed  at  shallow  depths,  81 
lenticular,  92 
pegmatitic,  90 

regionally  metamorphosed,  91 
replacement,  99 
syngenetic,  26 
Ores,  contact,  79,  88 
low  grade,  1 1 
oxidized,  169,  170 
primary,  69 

criteria  of,  73,  115 
decrease    in    value    of,    in 

depth,  123 
depth  to  which  persistent, 

72,  81,  124,  139 
refractory,  8 
residual,  73 
secondary,  173 

criteria  of,  174 
minerals  of,  176 

Ore  reserves,  calculation  of,  19,  24 
Ore-shoots,  distinction  between  pri- 
mary and  secondary,  115 
primary,  24,  114 

at  tops  of  anticlines,  136 
deep-seated,  24,  124,  143 
due  to  impounding,  130 
due  to  intersections,  127 
due  to  open  spaces,  127 
factors  determining,  114 
influence  of  wall  rocks  on, 

136 

irregularity  of,  114,  118 
lenticular,  120,  144 
overlapping,  120 


220 


INDEX 


Ore-shoots,  primary,  shapes  of,  118 
structure,  influence  of,  on, 

125 

terms  used  to  describe,  116 
residual,  171 
secondary,  24,  173,  176 

effect  of  porosity  on,  179 
effect  of  primary  mineral- 
ization on,  179 
effect  of  structure  on,  178 
effect  of  wall  rocks  on,  178 
effect  of  water  level  on,  178 
lenticular,  92 
shapes  of,  190 
Ouary,  Colo.,  90,  13Q,  132 
Outcrops,  196 
casts  in,  200 
composition  of,  201 
conditions  of  indicating  enrich- 
ments, 202 

of  contact  deposits,  205 
down  hill  creep  of,  198 
of  kaolinized  rocks,  204 
meandering  of,  197 
porosity  of,  200 

relation  of  length  to  depth,  196 
size  of  compared  with  size  of 

vein,  197 

of  superficial  deposits,  200,  205 
Overlapping  veins,  57 
Oxidation,  156 

minerals  resistant  to,  157 
irregularity  of,  162 
relation  to  topography  and 
water  level,  161 

Pachuca,  Mex.,  80,  195 
Paige,  Sidney,  96,  175 
Park  City,  Utah,  80,  210 
Pecos,  N.  M.,  90 
Pegmatitic  deposits,  90,  124 
Phillipsburg,  Mont.,  164 
Pilares  mine,  Sonora,  8,  207 


Pinos  Altos,  N.  M.,  96 

Pipes,  53 

Porphyry   copper  deposits,    8,    182, 

203 
Porosity,    effect    of,    on  secondary 

ores,  179 
Post-mineral   movements,    38,    125, 

197 

Preliminary  examinations,  1 
Pre-mineral  fissures,  38,  125 
Primary  alterations,  see  alterations, 

primary, 
minerals,  74,  75 

ore-shoots,  see  ore-shoots,  pri- 
mary, 
ores,  of  copper,  78,  79 

depth  of,  81 

of  gold,  77,  78 

of  lead,  80 

of  silver,  79,  80 

of  zinc,  81 
Promontorio  mine,  Durango,  Mex., 

187 

Propylitization,  83,  148 
Prospects,  examination  of,  3,  196 
exploration  of,  6,  9,  36,  40,  71, 

139 

Purington,  C.  W.,  143 
Pyrite,  precipitation  by,  138,  159 

Rand  mines,  South  Africa,  109,  118 

Randsburg,  Cal.,  7 

Ransome,  F.  L.,  28,  34,  38,  46,  50, 
60,77,79,80,  104,  116,119, 
124,  129, 150,  170, 181,  186, 
198,  209,  210,  212 

Ratio  of  concentration,  9 

Red  beds,  113,  143 

Red  Mountain,  Colo.,  53 

Refractory  ores,  8 

Regional  metamorphism,  91,  145 

Regularity  of  ore-deposits,  7,  22, 
81 


INDEX 


221 


Replacement,   conditions  favorable 
to,  103,  125,  138 

deposits,  100 

imitating  banding,  96 

selective,  101 

veins,  98 
Resampling,  19 
Residual  kernels,  165 

ores,  168 

ore-shoots,  171 
Resistant  minerals,  88,  201 
Reverse  faults,  34 
Rickard,  T.  A.,  12,  141 
Rico,  Colo.,  131 
Rio  Tinto,  Spain,  185 
Risk  of  mining,  5 
Roots  of  veins,  81 
Ruby  mine,  Mont.,  191 


Saddle  reefs,  134 
Sales,  R.  H.,  118 
Salting,  17 
Samples,  breaking,  12 

cutting  down,  12 

interval  between,  16 

marking,  13 

placing  the,  16 

shipment  of,  18 

size  of,  17 
Sampling,  12 

equipment  for,  12 

preparation  for,  14 
San  Antonio  de  la  Huerta,  Mexico, 

184 

San  Javier,  Mex.,  143 
San  Juan  Mtns.,  Colo.,  143 
San  Pedro,  N.  M.,  79,  211 
Santa  Eulalia,  Mex.,  107 
Schist,  alteration  of,  89 

veins  in,  67,  68 

Schistose  structure,  27,  121,  145 
Schwartz,  T.  E.,  53,  186 


Secondary    alterations,    see    altera- 
tions, secondary, 
ore-shoots,    see   ore-shoots,  sec- 
ondary. 

Sections,  geological,  37,  40 

Segregation   of    values,    indications 
of,  76 

Segregations,  magmatic,  86 

Selective  precipitation,  88,  139 

Sericitization,  82,  149 

Shafts,  sampling,  13 

Shasta  County,  Cal.,  78,  105 

Shear  zones,  52 

Sheeted  zones,  49 

Sheeting,  28,  34 

Sierra  Mojada,  Mex.,  102 

Sierra  Nevada,  deposits  of,  71 

Silicification  1 50 

Silver,  distribution  of,  by  oxidation, 

172 

enrichment  of,  185,  186 
primary  ores  of,  79,  80 

Silver  Lake,  Colo.,  54 

Silver  Peak,  Nev.,  86,  87 

Silverton,  Colo.,  50,  94,  120,  185,  212 

Size  of  ore-deposits,  11 

Smith,  C.  H.,  112 

Snowstorm  mine,  Ida., 

Solution,  enlargement  of  cavities  by, 
94 

Solution  breccias,  207,  208 

Specific  gravity,   determination 
20 

Spencer,  A.  C.,  30,  47,  56,  77 

Spurr,  J.  E.,  33,  80,  87,  126,  130, 141, 
187, 188, 191, 192,  212 

Step  faults,  35 

Stocks,  53 

Stockworks,  53 

Stoping  width,  20 

Stratification,  26 

Striations,  36 

Stringer  lodes,  50 


222 


INDEX 


Structural  geology,  26 
Structure,  gneissic,  27 

schistose,  27 
Surficial  deposits,  205 
Syncline,  31 
Syngenetic  deposits,  26 
Systems  of  related  veins,  57 

Tecoripa,  Mex.,  143 

Telluride,  Colo.,  118 

Terms  of  sale,  5,  6 

Throw,  33 

Title,  7 

Tomboy  mine,  Colo.,  56 

Tombstone,  Ariz.,  134 

Tonopah,  Nev.,  36,  41,  80,  84,  130, 

212 

Topography,  expression  of  faults  in, 
39,  199 

of  mineralization  in,  198 
Trail,  36 
Tularosa,  N.  M.,  113 

Union ville,  Nev.,  65 

United  Verde  mine,  Ariz.,  208 

Values,  minerals  indicating  a  segre- 
gation of,  76 

Van  Hise,  C.  R.,  26,  27,  30,  62,  95 
Veins  along  dikes,  48 

bed,  45 

blanket,  45 

branching,  54,  61,  81 

compound,  46,  47 

conjugate,  56,  57 

contact,  47 

deep-seated,  83 


Veins,     distinction    between    inter- 
calated and  fissure,  67 

fissure,  43 

follow  faults  of  small  displace- 
ment, 62 

formed  near  the  surface,  82 

gash,  44 

in  schist,  68 

intercalated,  67 

linked,  55 

movement  along,  63 

overlapping,  57 

persistence  of,  in  depth,  61 

pinching  and  swelling  of,  127 

pockety,  73 

relation  between  depth  and  num- 
ber of,  62 

replacement,  98,  136 

structure,  influence  of  country 
rock  on,  63 

systems  of,  57 
Virginia  City,  Nev.,  212 

Watson,  T.  L.,  206 
Weathering,  153 

minerals  resistant  to,  88 
Weed,  W.  H.,  67,  78,  79,  93, 107, 136, 

140,  156,  189,  190,  191 
Witwatersrand,  South  Africa,  109 

Yung,  M.  B.,  79 

Zinc,  enrichment  of,  194 

oxidized  ores  of,  170 

primary  ores  of,  81 
Zones  developed  by  surface  agen- 
cies, 163 


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